Validate calculated log canvas range

[ianthomas23]

Fixes #340, producing a sensible error message rather than an empty plot.

Previously a Canvas validated its x_range and y_range if they were explicitly specified by the user. But if they were calculated from the data limits, no validation occurred.

[codecov[bot]]

Codecov Report

Merging #1154 (340656f) into master (38afcc7) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1154      +/-   ##
==========================================
+ Coverage   85.35%   85.38%   +0.02%     
==========================================
  Files          35       35              
  Lines        7807     7812       +5     
==========================================
+ Hits         6664     6670       +6     
+ Misses       1143     1142       -1     

Impacted Files	Coverage Δ
datashader/core.py	88.05% <100.00%> (+0.28%)	:arrow_up:
datashader/data_libraries/dask.py	94.54% <100.00%> (+0.05%)	:arrow_up:
datashader/data_libraries/pandas.py	100.00% <100.00%> (ø)

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