Don't crash so hard when email settings are invalid

Premise: Users should be able to configure their site through AdminFE without ever touching the command line. This is a requirement for Tribes to be able to function.

Crashing hard for invalid email config makes this impossible. Instead we should just roll with the invalid config, warn the user on bootup, and let things fail when they fail. AdminFE should also never let invalid config be saved in the first place (see: admin-fe#192)

Rather than crashing at startup, I've converted those errors into logger warnings. I've also added a test to ensure that things still work even with the invalid config (emails aren't sent of course, but the user is still successfully registered).