[api-minor] Remove the image-related error message prefixes

[Snuffleupagus]

Other custom errors, based on BaseException, do not use such a format.

[Snuffleupagus]

/botio test

[moz-tools-bot]

From: Bot.io (Linux m4)

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Received

Command cmd_test from @Snuffleupagus received. Current queue size: 1

Live output at: http://54.241.84.105:8877/222cd369dac0c98/output.txt

[moz-tools-bot]

From: Bot.io (Windows)

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Received

Command cmd_test from @Snuffleupagus received. Current queue size: 1

Live output at: http://54.193.163.58:8877/5d582480e03c507/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

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Failed

Full output at http://54.241.84.105:8877/222cd369dac0c98/output.txt

Total script time: 26.94 mins

• Unit tests: Passed
• Integration Tests: Passed
• Regression tests: FAILED

  different ref/snapshot: 20
  different first/second rendering: 1

Image differences available at: http://54.241.84.105:8877/222cd369dac0c98/reftest-analyzer.html#web=eq.log

[moz-tools-bot]

From: Bot.io (Windows)

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Failed

Full output at http://54.193.163.58:8877/5d582480e03c507/output.txt

Total script time: 41.36 mins

• Unit tests: Passed
• Integration Tests: FAILED
• Regression tests: FAILED

  different ref/snapshot: 6

Image differences available at: http://54.193.163.58:8877/5d582480e03c507/reftest-analyzer.html#web=eq.log

[calixteman]

LGTM. Thank you.

[timvandermeij]

Thanks; using the actual exceptions rather than a message prefix overall seems much nicer!