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/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()));
     }