Added `VowsInternalError` to raise for any illegal behavior internal to PyVows
Import VowsInternalError for problems at runtime
Added `#FIXME` for a needed comment
`VowsParallelRunner.context_col` → `VowsParallelRunner.ctx_collection`

Never quite knew what `context_col` meant.  When I looked into it, I can only guess that “col” meant “collection”.

So, I renamed the variable in the hopes that its new name is more self-descriptive.
Added message to VowsInternalError encouraging bug reports
Close #48: Now catching internal errors when printing tracebacks

It’s not technically a “fix”…but it should alert users that there was a runtime error in PyVows, and not their own code (or tests).
Merge pull request #81 from Zearin/bug/48_Reporting_where_topic_error_is_None

Close #48: Now catching internal errors when printing tracebacks