Improve error catching and reporting in main script.

[relsqui]

The install script is pretty good at it now, the main script should be better.

[relsqui]

Current plan: go through and look for places I can check for errors instead of waiting for them to happen. For example: do EDITOR and PAGER exist (use which), are they executable, and various other permissions things. Will probably want a tidy function to error out into (something I can just pass a string to) although the set -e might mean I can just return nonzero from functions when that's what I want? possibly I should be doing this instead of that though.