Increase signal to noise in appveyor build output by reducing the MSVC warning count from 12 to 4 (12 is assuming the changes in #14086 are also implemented).

This makes it easier to spot errors or more important warnings in the verbose appveyor output.

See https://github.com/bitcoin/bitcoin/pull/14086#issuecomment-416610313 plus discussion for context.

src/rest.cpp

@@ -464,7 +464,7 @@ static bool rest_getutxos(HTTPRequest* req, const std::string& strURIPart)
                 oss >> fCheckMemPool;
                 oss >> vOutPoints;
             }
-        } catch (const std::ios_base::failure& e) {
+        } catch (const std::ios_base::failure&) {
             // abort in case of unreadable binary data
             return RESTERR(req, HTTP_BAD_REQUEST, "Parse error");
         }