Improve time index validation

[kmax12]

Minor fix converting an elif to if. In the current code, if the code paths related to index were taken, the elif to handle the time index wouldn't get called.

Our test case for this didn't check error message so it was passing. This has been updated. Also added another test related to another unchecked message.

[codecov-io]

Codecov Report

Merging #285 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #285      +/-   ##
==========================================
+ Coverage   94.44%   94.47%   +0.02%     
==========================================
  Files          71       71              
  Lines        7704     7708       +4     
==========================================
+ Hits         7276     7282       +6     
+ Misses        428      426       -2

Impacted Files	Coverage Δ
featuretools/entityset/entityset.py	93.82% <100%> (+0.18%)	⬆️
featuretools/tests/entityset_tests/test_es.py	99.36% <100%> (ø)	⬆️
featuretools/utils/gen_utils.py	70.58% <0%> (+1.96%)	⬆️

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