Validate input and lightly mount

[guarin]

Changes

• Validate input and lightly mount

This gives the user better feedback whether the input and lightly mounts were correctly passed to lightly-serve

How was it tested?

• Added unit tests
• Ran manual tests

[codecov]

Codecov Report

Attention: Patch coverage is 73.07692% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 82.21%. Comparing base (90c5d9f) to head (4efa35e).

Files	Patch %	Lines
lightly/cli/serve_cli.py	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1537      +/-   ##
==========================================
+ Coverage   82.19%   82.21%   +0.01%     
==========================================
  Files         144      144              
  Lines        6073     6116      +43     
==========================================
+ Hits         4992     5028      +36     
- Misses       1081     1088       +7     

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[japrescott]

looks amazing! Thanks! Just one UX question with the error msgs but already approved for you to make the decision