Fixed weird error message in core install if invalid platform is spcified

[cmaglie]

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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)
• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

Fixed an error message.

What is the current behavior?

$ arduino-cli core install asdasd:asdasd
Invalid argument passed: Found 0 platform for reference "asdasd:asdasd":

What is the new behavior?

$ arduino-cli core install asdasd:asdasd
Invalid argument passed: Platform 'asdasd:asdasd' not found

Does this PR introduce a breaking change, and is titled accordingly?

Other information

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (65915d8) 63.82% compared to head (8c55dd5) 63.82%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2309      +/-   ##
==========================================
- Coverage   63.82%   63.82%   -0.01%     
==========================================
  Files         214      214              
  Lines       19794    19793       -1     
==========================================
- Hits        12633    12632       -1     
  Misses       6073     6073              
  Partials     1088     1088              

Flag	Coverage Δ
unit	63.82% <100.00%> (-0.01%)	⬇️

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

Files Changed	Coverage Δ
arduino/errors.go	26.85% <100.00%> (-0.77%)	⬇️
internal/cli/arguments/reference.go	100.00% <100.00%> (ø)

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