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/walletdb.cpp

@@ -460,7 +460,7 @@ ReadKeyValue(CWallet* pwallet, CDataStream& ssKey, CDataStream& ssValue,
             {
                 ssValue >> hash;
             }
-            catch(...){}
+            catch (...) {}
 
             bool fSkipCheck = false;
 
@@ -664,7 +664,7 @@ DBErrors CWalletDB::LoadWallet(CWallet* pwallet)
         }
         pcursor->close();
     }
-    catch (boost::thread_interrupted) {
+    catch (const boost::thread_interrupted&) {
         throw;
     }
     catch (...) {
@@ -757,7 +757,7 @@ DBErrors CWalletDB::FindWalletTx(CWallet* pwallet, vector<uint256>& vTxHash, vec
         }
         pcursor->close();
     }
-    catch (boost::thread_interrupted) {
+    catch (const boost::thread_interrupted&) {
         throw;
     }
     catch (...) {
@@ -878,7 +878,7 @@ bool BackupWallet(const CWallet& wallet, const string& strDest)
 #endif
                     LogPrintf("copied wallet.dat to %s\n", pathDest.string());
                     return true;
-                } catch(const filesystem::filesystem_error &e) {
+                } catch (const filesystem::filesystem_error& e) {
                     LogPrintf("error copying wallet.dat to %s - %s\n", pathDest.string(), e.what());
                     return false;
                 }