Improve diagnostic messages for syntactic issues in expressions and actions

[cwecht]

Right now, errors during parsing of exceptions and actions are indicated like this:

ERROR: invalid action: /add_two_ints/request/a = 1

This diagnostic message is not particularly helpful.

1. It only tells us that something is wrong here, but not what (missing semicolon to form a proper assignment).
2. It only shows the invalid action/condition but does not give context e.g., at which line in which file this error occurred.