Do not panic on unexpected key format

[pontus]

We used to panic on non-PEM-encoded keys, don't do that.

Also, some PEM-areas with less data than expected would also cause panics, fixes that as well.

[codecov-commenter]

Codecov Report

Merging #85 (48a0c2e) into master (c6719a4) will increase coverage by 0.69%.
The diff coverage is 100.00%.

❗ Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
+ Coverage   65.54%   66.24%   +0.69%     
==========================================
  Files           6        6              
  Lines        1164     1176      +12     
==========================================
+ Hits          763      779      +16     
+ Misses        281      278       -3     
+ Partials      120      119       -1     

Flag	Coverage Δ
unittests	66.24% <100.00%> (+0.69%)	⬆️

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

Files Changed	Coverage Δ
keys/keys.go	62.24% <100.00%> (+2.60%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more