Prevent prefilter from crashing IPython

[minrk]

Since user code can be executed in the prefilter step (e.g. via Aliases), catch and print those errors like normal code.

I could also be less aggressive and only catch AliasErrors.

Closes #216

[rkern]

Why except SystemExit: instead of except Exception:?

[minrk]

Because I managed to commit while I was checking that the test I added failed when the exception wasn't caught.

Fixed now.

[takluyver]

Looks good to me. I'll test it later.

[takluyver]

That's working here. Is it worth catching the specific case (wrong number of arguments for an alias) and displaying a short error message, rather than the full traceback?

[minrk]

If we want IPython to die at every other error, I can only catch the AliasError. I don't actually think that's a good idea, but it would conform with current practices of making IPython crash and exit at the first sign of unrecognized trouble.

I think IPython should only crash when something is unrecoverable, not at every possible bug in IPython, but that's something of a separate issue.

[rkern]

I think he's suggesting that you catch the AliasError specifically to make a more focused error message, then catch the rest of the exceptions with the general except Exception:

[takluyver]

As Robert says - I'm not suggesting you remove the general except handler. Just that, in addition, I'd prefer to avoid a traceback of IPython internals for a simple mistake in user code. E.g. for magic functions, if the user's command is wrong in some way, we often just print a single line explanation of the problem, and return from the function.

[minrk]

Okay, now the message is just the short 'ERROR: ' on AliasErrors, but all exceptions are caught.

[takluyver]

OK, looks good to me.