Error cascade

[xxxserxxx]

This version (which can be merged on top of, or instead of, report_errors) replaces ExitFail() in the top-level functions, such that errors are passed up to the main function and reported there. Long-term, ExitFail() would ideally be removed entirely; by exiting deep in the stack, it prevents calling functions the opportunity to handle and clean up after errors. However, this PR focuses on having consistent behavior in all of the top-level commands by using RunE() wherever there can be failures.

[xxxserxxx]

All of these PRs are based on different branches, and they often conflict. If you're going to pull any of them, I recomend pulling issue-69 and user-keys-master first, then letting me know which others you do (or don't) want to merge, and letting me resolve the conflicts.

[naggie]

Thanks @xxxserxxx -- I will get the user-keys-master branch merged first I think. A useful change and helps @botto 's plight of separating the cli into a lib. @botto please take a look too.

[xxxserxxx]

Rebased naggie/master onto this so it's a clean merge again.