Minor changes in catch command (mainly style)
Fill up line up to 80 chars
Consistent command name and file name
Fixes #93

Up/Down/Frame commands could still fail if Byebug's calculated stack
size does not match the real one, but it should be fine because:
* I think that issue is almost totally gone.
* The user will usually use those commands after checking the stack with
the `backtrace` command and that command is still making sure only valid
frames are displayed.
Whitespace
Warning doesn't make sense in post_mortem mode
Do nothing in post_mortem when no exception risen
Fix typo