Allow stricter permissions but not looser; Change default permissions to 0400; Correct and improve `unprotected key file' message.

[SBell6hf]

This pull request:

• Allows stricter permissions but not looser
• Changes default permissions to 0400
• Changes permission denied => unprotected key file `FILENAME': file permissions too open
• Fixes: Improve error message when file permissions deny access to a file #51, Certstrap allows looser permissions but not stricter, which can lead to security issues #136, Default permissions too open #137

[CLAassistant]

All committers have signed the CLA.

[jdtw]

Looks like this test is failing: https://github.com/square/certstrap/blob/master/depot/depot_test.go#L124-L126

[SBell6hf]

​

[jdtw]

Thanks so much for filing these issues! I found the tests confusing, so I refactored them and fixed the logic error in #141. After that change, I propose we split this PR into two:

1. Make the error string more descriptive.
2. Change the default file permissions if needed (I left a comment on the issue).