FIX: Allow verify_integrity to work on general t_column. Improve messages.

1. A simple fix: change hard-coded `'frame'` to `t_column` as pointed out in #152. Add associated test to guard against reintroducing hard-coded `'frame'` in the future.

2. Previously, any malformed particle labeling due some unforeseen bug in linking raised either an `UnknownLinkingError` with an informative explanation or a totally uninformative `AssertionError`. All of the latter have been replaced by the former.

#151 technically changes the API and should wait for 0.3, but this can go into 0.2.3.