Improve Amazon SES code error handling behavior

Summary:
Fixes T10728. Fixes T10476. SES uses third-party code with unique, creative ideas about error handling.

• Make the error handling behavior more correct, so it doesn't try to use undefined variables.
• Simplify the error handling behavior (throw exceptions sooner, remove redundant code).
• Explicitly test for -smtp misconfigurations. These can arise if you read the wrong column out of the table in the AWS docs, as in T10728.
• Explicitly test for SimpleXML, to catch T10476 before it does damage.

Test Plan:

• Configured SES to use a bogus SMTP endpoint.
• Faked past the SMTP check, hit sane error on the connection.
• Undid faking, hit immediate hard stop on the STMP check.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10476, T10728

Differential Revision: https://secure.phabricator.com/D15632