Add mock error case for PipelineBase.graph

[jeremyliweishih]

Patch for #549.

[codecov]

Codecov Report

Merging #665 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #665      +/-   ##
==========================================
+ Coverage   98.97%   99.02%   +0.04%     
==========================================
  Files         135      135              
  Lines        4702     4709       +7     
==========================================
+ Hits         4654     4663       +9     
+ Misses         48       46       -2     

Impacted Files	Coverage Δ
evalml/tests/pipeline_tests/test_graphs.py	100.00% <100.00%> (ø)
evalml/pipelines/pipeline_base.py	98.59% <0.00%> (+0.93%)	⬆️

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[dsherry]

Love it. I like the same-line changelog entries too 👍