fix: sketch name validation error messages

[Bikappa]

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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
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What kind of change does this PR introduce?

Follow up on #2051. Fixing the error messages returned when validating the sketch name

What is the current behavior?

Errors are difficult to understand

What is the new behavior?

Errors are more human readable

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

Other information

[codecov]

Codecov Report

Base: 36.67% // Head: 36.76% // Increases project coverage by +0.08% 🎉

Coverage data is based on head (b1aacff) compared to base (2b8d6d7).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2059      +/-   ##
==========================================
+ Coverage   36.67%   36.76%   +0.08%     
==========================================
  Files         228      228              
  Lines       19384    19383       -1     
==========================================
+ Hits         7110     7126      +16     
+ Misses      11442    11429      -13     
+ Partials      832      828       -4     

Flag	Coverage Δ
unit	36.76% <100.00%> (+0.08%)	⬆️

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

Impacted Files	Coverage Δ
commands/sketch/new.go	68.42% <100.00%> (-0.81%)	⬇️
arduino/cores/packagemanager/package_manager.go	68.19% <0.00%> (+0.72%)	⬆️
arduino/errors.go	4.44% <0.00%> (+0.74%)	⬆️
arduino/monitor/monitor.go	47.36% <0.00%> (+6.31%)	⬆️

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