Raise exception if prune returns no conformers

[t-young31]

Raises a more helpful exception in e.g #268

Questions

• Is this behaviour intuitive? Certainly useful for debugging but are there circumstances where you'd want to call prune and allow an empty result?

[codecov[bot]]

Codecov Report

Merging #271 (7529e0a) into v1.4.0 (fe54c19) will increase coverage by 0.42%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           v1.4.0     #271      +/-   ##
==========================================
+ Coverage   96.81%   97.23%   +0.42%     
==========================================
  Files         197      197              
  Lines       21006    21032      +26     
==========================================
+ Hits        20336    20450     +114     
+ Misses        670      582      -88     

Flag	Coverage Δ
unittests	97.23% <100.00%> (+0.42%)	:arrow_up:

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

Impacted Files	Coverage Δ
autode/conformers/conformers.py	100.00% <100.00%> (ø)
tests/test_conformers.py	99.53% <100.00%> (+0.02%)	:arrow_up:

... and 9 files with indirect coverage changes