Update mailer.py with some improvements

[ShadowJonathan]

Description

Fixes #393, making sure the mailer works

Changes

• Remove 2 print statements of exceptions that prevented me from finding this bug in the first place; just let python print the traceback
• Remove one dependency statement for jinja2, it is now built-in

Checklist

• Pull request is limited to a single purpose
• Code style/formatting is consistent
• All existing tests are passing
• Added new tests related to change
• No unnecessary whitespace changes

Collaboration
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generally has the authority to decide if other users can commit to the
pull request's compare branch. If the pull request author wants greater
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(that is, anyone with push access to the upstream repository) permission
to commit to the pull request's compare branch

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[satterly]

Thanks for the improvements! A few issues need addressing before this can be merged:

1. Bug: stale variable reference — x.strip() should be c.strip() (or just c, since rule_contacts values are already stripped). The old list comprehension variable x no longer exists, so this will raise a NameError at runtime whenever a matching non-exclude rule is found.

2. break on exclude match changes behavior — The new break after an exclude rule means only the first matching exclude rule applies and no further rules are evaluated. Is this intentional? If so, please document the rationale.

3. Main-thread exception handling removed — Removing the except Exception blocks in main() means main-thread exceptions now produce a full traceback to stderr instead of a clean message + exit. This is a behavior change worth noting.

Please fix #1 at minimum — it's a runtime crash.