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Update GPGMail introduction.
[NEW] Proper error handling of encryption or signing failures.

- Show proper error messages instead of the SMIME_CANT_SIGN_MESSAGE
- Add error output from GPG if available to make it easier for users to
  specify what the problem was on our support platform.
Sign and encrypt default to NO unless overridden.
Merge branch 'experimental' of github.com:GPGTools/GPGMail into experimental