Improve errorcodes.py so it captures multiline messages

The output of buildscripts/errorcodes.py currently contains a large number of missing or truncated messages. At least 400 are visible here:

http://docs.10gen.cc/kernel/master/errors.html

Not being able to see these messages in the errors.md output prevents us from providing a full audit as requested in DOCS-1519, and makes it difficult to improve our messages.

Many of these missing messages are caused by errorcodes.py naively walking the source files line-by-line, without lookahead. Let's fix this.