Vis: Handle missing data more gracefully

[dweindl]

If there are no measurements for a plot, don't try to plot measurements.

Also provides more informative messages if plotting fails.

[codecov-commenter]

Codecov Report

Merging #175 (7f1fdf8) into develop (ff1bdde) will decrease coverage by 0.05%. The diff coverage is 60.00%.

@@             Coverage Diff             @@
##           develop     #175      +/-   ##
===========================================
- Coverage    78.52%   78.46%   -0.06%     
===========================================
  Files           30       30              
  Lines         2649     2652       +3     
  Branches       605      605              
===========================================
+ Hits          2080     2081       +1     
- Misses         410      412       +2     
  Partials       159      159              

Impacted Files	Coverage Δ
petab/visualize/plotter.py	78.78% <60.00%> (-0.85%)	:arrow_down:

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