Error handling and messages

[EivindArvesen]

Handle some edge cases, probably needs a couple of error messages as well.

• What if files dissappear at unexpected points?
• What if users try to name project something that exists (e.g. temp-file) or is reserved or starts with dot?

Related to issue #12.

Some relevant lines can be seen in adbf77c.

[nikklassen]

I'm not sure that this belongs here, but it would be handy to warn the user if $EDITOR is empty. It took me a while to debug the error that arises from that situation.

[EivindArvesen]

In any case, it certainly needs fixing!

[EivindArvesen]

@nikklassen : Thanks for the heads-up! Added a test to check whether $EDITOR is set before add, copy and edit actions.

[nikklassen]

Sweet, thanks!