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 (...) {