Add friendly message for invalid query and 0 results

[gdubicki]

Add friendly message for invalid query and 0 results

instead of, respectively, showing the raw HTTP 400 without
the specific reason and not showing anything.

Also:
* show the number of results when they are > 0,
* use fixed-width font to query textarea and the error
  message (if there is one), to point to error in the
  the query precisely,
* remove the nonsense "cancel" button, make the "submit"
  look active,

[gdubicki]

This is how it looks like:

• zero results:

• error in query:

• normal result:

[gdubicki]

Originally I wanted to finish #429 and this was supposed to be just a warmup, but it turned out to be enough work to release it as a separate PR.

[codecov]

Codecov Report

Merging #653 (c38c15b) into master (4c2f1d9) will increase coverage by 1.70%.
The diff coverage is 90.62%.

❗ Current head c38c15b differs from pull request most recent head 333950f. Consider uploading reports for the commit 333950f to get more accurate results

@@            Coverage Diff             @@
##           master     #653      +/-   ##
==========================================
+ Coverage   81.01%   82.72%   +1.70%     
==========================================
  Files          12       12              
  Lines         885      897      +12     
==========================================
+ Hits          717      742      +25     
+ Misses        168      155      -13     

Impacted Files	Coverage Δ
puppetboard/app.py	78.25% <86.95%> (+2.43%)	⬆️
puppetboard/utils.py	93.87% <100.00%> (+0.47%)	⬆️

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