get_results throws exception when dir doesn't exist

[heavelock]

I create a separate pull request to extract this feature from PR#101.

This just checks if directory created out from provided station names exists. In case it doesn't, an exception is being thrown.

There is also added a test for that exception throwing.

[codecov]

Codecov Report

Merging #107 into master will decrease coverage by <.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master     #107      +/-   ##
==========================================
- Coverage   73.44%   73.43%   -0.01%     
==========================================
  Files          17       17              
  Lines        2478     2481       +3     
==========================================
+ Hits         1820     1822       +2     
- Misses        658      659       +1

Impacted Files	Coverage Δ
msnoise/api.py	61.07% <75%> (+0.02%)	⬆️

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