Unify messaging logic

[filipsch]

Soon, there will be two forms of message 'logging' or generation in protowhat:

• Through the history attribute on State (used in check_field() and check_node())
• Through the messages attribute (used by all of sqlwhat's result-checking functions.

For now, it's not too big of a problem, but it could become one in the future. Ideally, there's a unified system of building 'expand messages' like in pythonwhat, that point the student to the right bit of code, and a hint at what is wrong or how things can be fixed.