Don't allow catch command in control mode

Related to #92. Catch command is not allow anymore in control mode. It
was broken and I don't think it's worth fixing it. When loading the init
script, there's not an available context with associated
file/line/binding so we can not check whether the class to be catched is
defined or not. I can think of some workarounds but this is exactly how
"set post_mortem" is meant to be used so I'm just banning it.
Let the interface deal with verbosity

It is simpler and eliminates duplicated output.
Fix bug when loading byebugrc file.

Previously, if an invalid command was found, loading would be aborted.
Now an error is printed but the rest of the settings are loaded.
Make sure .byebugrc's don't interfere with tests