make all catch() arguments const

- I saw this on http://en.cppreference.com/w/cpp/language/try_catch and
  thought it would be a good idea
- also unify used format to better be able to search for exception
  uses in our codebase

src/qt/bitcoin.cpp

@@ -173,7 +173,7 @@ private:
     boost::thread_group threadGroup;
 
     /// Pass fatal exception message to UI thread
-    void handleRunawayException(std::exception *e);
+    void handleRunawayException(const std::exception *e);
 };
 
 /** Main Bitcoin application object */
@@ -240,7 +240,7 @@ BitcoinCore::BitcoinCore():
 {
 }
 
-void BitcoinCore::handleRunawayException(std::exception *e)
+void BitcoinCore::handleRunawayException(const std::exception *e)
 {
     PrintExceptionContinue(e, "Runaway exception");
     emit runawayException(QString::fromStdString(strMiscWarning));
@@ -260,7 +260,7 @@ void BitcoinCore::initialize()
             StartDummyRPCThread();
         }
         emit initializeResult(rv);
-    } catch (std::exception& e) {
+    } catch (const std::exception& e) {
         handleRunawayException(&e);
     } catch (...) {
         handleRunawayException(NULL);
@@ -277,7 +277,7 @@ void BitcoinCore::shutdown()
         Shutdown();
         qDebug() << __func__ << ": Shutdown finished";
         emit shutdownResult(1);
-    } catch (std::exception& e) {
+    } catch (const std::exception& e) {
         handleRunawayException(&e);
     } catch (...) {
         handleRunawayException(NULL);
@@ -551,7 +551,7 @@ int main(int argc, char *argv[])
     }
     try {
         ReadConfigFile(mapArgs, mapMultiArgs);
-    } catch(std::exception &e) {
+    } catch (const std::exception& e) {
         QMessageBox::critical(0, QObject::tr("Bitcoin Core"),
                               QObject::tr("Error: Cannot parse configuration file: %1. Only use key=value syntax.").arg(e.what()));
         return false;
@@ -628,7 +628,7 @@ int main(int argc, char *argv[])
         app.exec();
         app.requestShutdown();
         app.exec();
-    } catch (std::exception& e) {
+    } catch (const std::exception& e) {
         PrintExceptionContinue(&e, "Runaway exception");
         app.handleRunawayException(QString::fromStdString(strMiscWarning));
     } catch (...) {

src/qt/intro.cpp

@@ -95,7 +95,7 @@ void FreespaceChecker::check()
                 replyMessage = tr("Path already exists, and is not a directory.");
             }
         }
-    } catch(fs::filesystem_error &e)
+    } catch (const fs::filesystem_error&)
     {
         /* Parent directory does not exist or is not accessible */
         replyStatus = ST_ERROR;
@@ -180,7 +180,7 @@ void Intro::pickDataDirectory()
             try {
                 TryCreateDirectory(GUIUtil::qstringToBoostPath(dataDir));
                 break;
-            } catch(fs::filesystem_error &e) {
+            } catch (const fs::filesystem_error&) {
                 QMessageBox::critical(0, tr("Bitcoin Core"),
                     tr("Error: Specified data directory \"%1\" cannot be created.").arg(dataDir));
                 /* fall through, back to choosing screen */

src/qt/paymentrequestplus.cpp

@@ -181,8 +181,7 @@ bool PaymentRequestPlus::getMerchant(X509_STORE* certStore, QString& merchant) c
         }
         // TODO: detect EV certificates and set merchant = business name instead of unfriendly NID_commonName ?
     }
-    catch (SSLVerifyError& err)
-    {
+    catch (const SSLVerifyError& err) {
         fResult = false;
         qWarning() << "PaymentRequestPlus::getMerchant : SSL error: " << err.what();
     }

src/qt/rpcconsole.cpp

@@ -180,7 +180,7 @@ void RPCExecutor::request(const QString &command)
 
         emit reply(RPCConsole::CMD_REPLY, QString::fromStdString(strPrint));
     }
-    catch (json_spirit::Object& objError)
+    catch (const json_spirit::Object& objError)
     {
         try // Nice formatting for standard-format error
         {
@@ -188,12 +188,12 @@ void RPCExecutor::request(const QString &command)
             std::string message = find_value(objError, "message").get_str();
             emit reply(RPCConsole::CMD_ERROR, QString::fromStdString(message) + " (code " + QString::number(code) + ")");
         }
-        catch(std::runtime_error &) // raised when converting to invalid type, i.e. missing code or message
+        catch (const std::runtime_error&) // raised when converting to invalid type, i.e. missing code or message
         {   // Show raw JSON object
             emit reply(RPCConsole::CMD_ERROR, QString::fromStdString(write_string(json_spirit::Value(objError), false)));
         }
     }
-    catch (std::exception& e)
+    catch (const std::exception& e)
     {
         emit reply(RPCConsole::CMD_ERROR, QString("Error: ") + QString::fromStdString(e.what()));
     }