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/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 */