Errors.hs: improve field names for ConfigVersionMismatch

Change {exp,act}Code to {exp,act}Ver, which gives a better idea that
the integer fields represent version numbers.

Also:

  - errors.py: update OpPrereqError's docstring to note that an error
    code is always expected as the second argument (it was previously
    optional).

Signed-off-by: Dato Simó <dato@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>