refactor: adds custom exceptions

[gnikit]

This is a first attempt at the problem.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 84.74576% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 88.03%. Comparing base (a5fc5ed) to head (2c4273d).

Files	Patch %	Lines
fortls/langserver.py	57.14%	9 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #380      +/-   ##
==========================================
+ Coverage   87.99%   88.03%   +0.04%     
==========================================
  Files          35       36       +1     
  Lines        4788     4799      +11     
==========================================
+ Hits         4213     4225      +12     
+ Misses        575      574       -1     

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