Improve error message for invalid file name patterns

[DysphoricUnicorn]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no
• existing test has been updated

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

Description

This error message felt really confusing to me and even while reading the documentation, it was unclear how I could get my entries to be stored in a sub directory.
I added this little hint to the error message so that others may not run into the same issue.

[lukastaegert]

Good idea, I like these kind of contributions 👍

[codecov]

Codecov Report

Merging #4556 (e6f4e7a) into master (2e2a659) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4556   +/-   ##
=======================================
  Coverage   98.88%   98.89%           
=======================================
  Files         208      208           
  Lines        7282     7332   +50     
  Branches     2081     2094   +13     
=======================================
+ Hits         7201     7251   +50     
  Misses         26       26           
  Partials       55       55           

Impacted Files	Coverage Δ
src/utils/renderNamePattern.ts	100.00% <ø> (ø)
cli/run/batchWarnings.ts	99.18% <0.00%> (-0.82%)	⬇️
src/Bundle.ts	100.00% <0.00%> (ø)
src/utils/transform.ts	100.00% <0.00%> (ø)
cli/run/waitForInput.ts	100.00% <0.00%> (ø)
src/utils/FileEmitter.ts	100.00% <0.00%> (ø)
src/utils/pluginUtils.ts	100.00% <0.00%> (ø)
src/utils/PluginDriver.ts	100.00% <0.00%> (ø)
src/utils/PluginContext.ts	100.00% <0.00%> (ø)
src/ast/nodes/MetaProperty.ts	100.00% <0.00%> (ø)
... and 2 more

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