Augment error message content

[alexeagle]

Users should be able to add a mapping in their checked-in aspect config file, from something to match in the bazel stdout (maybe a regex) to an additional error message to print.

This allows use cases like

• "anytime there's an error about protoc failing to compile, show a go/ link to my internal company knowledge base"
• "remind users to run the //:update_build_files target in my repo"

Note that we may have to distinguish both a user-preferences file and a config file checked into the project and shared with other engineers.