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/init.cpp

@@ -778,7 +778,7 @@ bool AppInit2(boost::thread_group& threadGroup)
             try {
                 boost::filesystem::rename(pathDatabase, pathDatabaseBak);
                 LogPrintf("Moved old %s to %s. Retrying.\n", pathDatabase.string(), pathDatabaseBak.string());
-            } catch(boost::filesystem::filesystem_error &error) {
+            } catch (const boost::filesystem::filesystem_error&) {
                  // failure is ok (well, not really, but it's not worse than what we started with)
             }
 
@@ -931,7 +931,7 @@ bool AppInit2(boost::thread_group& threadGroup)
                 filesystem::create_hard_link(source, dest);
                 LogPrintf("Hardlinked %s -> %s\n", source.string(), dest.string());
                 linked = true;
-            } catch (filesystem::filesystem_error & e) {
+            } catch (const filesystem::filesystem_error& e) {
                 // Note: hardlink creation failing is not a disaster, it just means
                 // blocks will get re-downloaded from peers.
                 LogPrintf("Error hardlinking blk%04u.dat : %s\n", i, e.what());
@@ -1008,7 +1008,7 @@ bool AppInit2(boost::thread_group& threadGroup)
                     strLoadError = _("Corrupted block database detected");
                     break;
                 }
-            } catch(std::exception &e) {
+            } catch (const std::exception& e) {
                 if (fDebug) LogPrintf("%s\n", e.what());
                 strLoadError = _("Error opening block database");
                 break;