chore(slack): Removed Expire Url Error Check

[iamrajjoshi]

metric to capture the expired url error has existed for two weeks, but we have never hit that error. here, i am cleaning up the error util and some of the exception capturing since we don't need to worry about it.

also decided to try out the capture_message from our sdk to notify us via a sentry issue if there is a new slack error that we haven't handled.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

:white_check_mark: All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/integrations/slack/notifications.py	50.00%	1 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #76352   +/-   ##
=======================================
  Coverage   78.17%   78.17%           
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  Files        6900     6900           
  Lines      307066   307058    -8     
  Branches    52894    52890    -4     
=======================================
- Hits       240037   240031    -6     
+ Misses      60609    60604    -5     
- Partials     6420     6423    +3