fix: pass in slack error messages properly

[eschutho]

SUMMARY

This fixes a logging issue where an error.message wasn't being properly passed in.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Before:

After:

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #22727 (f7d2680) into master (5399365) will decrease coverage by 11.07%.
The diff coverage is 68.62%.

❗ Current head f7d2680 differs from pull request most recent head b9c95b1. Consider uploading reports for the commit b9c95b1 to get more accurate results

@@             Coverage Diff             @@
##           master   #22727       +/-   ##
===========================================
- Coverage   67.07%   56.01%   -11.07%     
===========================================
  Files        1869     1869               
  Lines       71591    71609       +18     
  Branches     7822     7822               
===========================================
- Hits        48018    40110     -7908     
- Misses      21544    29472     +7928     
+ Partials     2029     2027        -2     

Flag	Coverage Δ
hive	52.58% <37.83%> (-0.02%)	⬇️
mysql	?
postgres	?
presto	52.48% <37.83%> (-0.02%)	⬇️
python	58.26% <64.86%> (-23.05%)	⬇️
sqlite	?
unit	51.49% <48.64%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset-frontend/src/middleware/asyncEvent.ts	78.18% <0.00%> (ø)
...uperset-frontend/src/pages/ChartCreation/index.tsx	61.11% <ø> (ø)
...uperset-frontend/src/pages/ChartList/ChartCard.tsx	52.17% <ø> (ø)
superset-frontend/src/pages/ChartList/index.tsx	54.61% <ø> (ø)
...set-frontend/src/views/CRUD/welcome/ChartTable.tsx	62.50% <ø> (ø)
superset/db_engine_specs/base.py	75.77% <0.00%> (-13.50%)	⬇️
superset/db_engine_specs/impala.py	38.46% <ø> (-6.42%)	⬇️
superset/reports/notifications/slack.py	33.33% <0.00%> (-58.03%)	⬇️
superset-frontend/src/views/routes.tsx	55.00% <33.33%> (ø)
superset/views/dashboard/views.py	58.82% <50.00%> (-7.46%)	⬇️
... and 310 more

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