Add error message when target entity does not exist

[CJStadler]

Also improve error message for EntitySets without an id when there is no
entity with the given name.

Resolves #466.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #520 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #520      +/-   ##
=========================================
+ Coverage   96.09%   96.1%   +<.01%     
=========================================
  Files         108     108              
  Lines        8881    8898      +17     
=========================================
+ Hits         8534    8551      +17     
  Misses        347     347

Impacted Files	Coverage Δ
featuretools/synthesis/deep_feature_synthesis.py	96.77% <100%> (+0.03%)	⬆️
...uretools/tests/entityset_tests/test_es_metadata.py	96.9% <100%> (+0.13%)	⬆️
featuretools/entityset/entityset.py	95.05% <100%> (+0.01%)	⬆️
...ols/tests/dfs_tests/test_deep_feature_synthesis.py	98.58% <100%> (+0.03%)	⬆️

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[rwedge]

Looks good