Code review comment for lp:~parthm/bzr/300062-invalid-pattern-warnings

So, lets go with an error based approach for now, for the following reasons:
 - its a very small change compared to the current code (wrap the
regex compiler to know its taking a list in and print the errorneous
pattern out)
 - it seems genuinely better for some use cases like add
 - we can add a warning based one later if this feels insufficient.

In general I do think its better to stop early and cleanly in commands
that can output lots of stuff - if we show a lot then users will often
miss the fine print.

I'll review the error based one again to move things forward.