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/test/dbwrapper_tests.cpp

@@ -262,7 +262,7 @@ struct StringContentsSerializer {
                 try {
                     READWRITE(c);
                     str.push_back(c);
-                } catch (const std::ios_base::failure& e) {
+                } catch (const std::ios_base::failure&) {
                     break;
                 }
             }