Remove punctuation at the end of 2 error strings to fix lint

[cyli]

Master has been failing with the following:

nat/parse.go:36:27: error strings should not be capitalized or end with punctuation or a newline
tlsconfig/config.go:166:26: error strings should not be capitalized or end with punctuation or a newline

All of the errors we return seem to be capitalized, and somehow that doesn't trigger the lint errors despite it saying that error strings should not be capitalized, so it seems like punctation that's the problem.

Alternately if we wanted to lower-case all of these errors I'll be happy to do that too.

[aaronlehmann]

I would lower-case the errors and reformat them so they are not written in sentence form. For the second error message, I think the ideal solution would be to check somehow for an encrypted key and have a specific error message for that case, but otherwise we could change it to something like error reading X509 key pair - make sure the key is not encrypted (cert: %q, key: %q): %v.