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/rpcrawtransaction.cpp

@@ -545,7 +545,7 @@ Value signrawtransaction(const Array& params, bool fHelp)
             ssData >> tx;
             txVariants.push_back(tx);
         }
-        catch (const std::exception &) {
+        catch (const std::exception&) {
             throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "TX decode failed");
         }
     }