sem, pragmas: fix fatal pragma

[starsiderfox]

Summary

Using {.fatal:"...".} did not show any message to the user and it
didn't stop compilation.
This commit fixes both issues.

Details

• adSemFatalError now carries a message just like
  adSemCustomUserError .
• All reports of fatal severity abort compilation.
• Testament now also consider fatal errors to be errors.
• A simple test case has been included.
• isCompilerFatal has been removed as it seems it lost its purpose.
  The check for rextCmdRequiresFile seems to be unnecessary. A fatal
  report is produced elsewhere (the only place where
  rextCmdRequiresFile is used) before this function is ever called, so
  it was redundant and it doesn't break existing code.

[zerbina]

This is a very clean and concise fix. Thank you!

There are two small things that need adjustment, but otherwise this looks good.

[zerbina]

Also, please make sure the PR message follows the guidelines. The first line works well as the summary; the details section should describe what changed internally (i.e., adSemFatalError now carries an error message, and sem reports with fatal severity are considered fatal errors).

[zerbina]

Apart from isFatalSeverity (refer to the comment I left regarding it), this PR is good to be merged.

[zerbina]

Looks good from my side, thank you.

[saem]

LGTM

[zerbina]

/merge

[github-actions]

Merge requested by: @zerbina

Contents after the first section break of the PR description has been removed and preserved below:

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