improve error message with readable HugeType

[javeme]

Change-Id: I633eee63fe9458cdc910f977fccec8fd0109f1d2

[codecov]

Codecov Report

Merging #546 into master will increase coverage by 0.02%.
The diff coverage is 82.75%.

@@             Coverage Diff              @@
##             master     #546      +/-   ##
============================================
+ Coverage     70.09%   70.11%   +0.02%     
- Complexity     3410     3414       +4     
============================================
  Files           216      216              
  Lines         16503    16516      +13     
  Branches       2360     2361       +1     
============================================
+ Hits          11567    11580      +13     
- Misses         3678     3679       +1     
+ Partials       1258     1257       -1

Impacted Files	Coverage Δ	Complexity Δ
...aidu/hugegraph/backend/tx/AbstractTransaction.java	68.11% <0%> (-0.5%)	37 <0> (ø)
...e/src/main/java/com/baidu/hugegraph/HugeGraph.java	70.88% <0%> (ø)	61 <0> (ø)	⬇️
...u/hugegraph/schema/builder/VertexLabelBuilder.java	87.37% <100%> (+0.06%)	64 <3> (ø)	⬇️
.../baidu/hugegraph/backend/tx/SchemaTransaction.java	87.89% <100%> (+0.12%)	62 <4> (ø)	⬇️
...u/hugegraph/schema/builder/PropertyKeyBuilder.java	87.61% <100%> (+0.11%)	36 <2> (ø)	⬇️
...du/hugegraph/schema/builder/IndexLabelBuilder.java	86.54% <100%> (+0.24%)	43 <2> (+1)	⬆️
...c/main/java/com/baidu/hugegraph/type/HugeType.java	88.09% <100%> (+0.29%)	29 <1> (+1)	⬆️
...java/com/baidu/hugegraph/schema/SchemaManager.java	97.5% <100%> (+0.06%)	22 <0> (ø)	⬇️
...om/baidu/hugegraph/exception/ExistedException.java	100% <100%> (ø)	2 <1> (+1)	⬆️
...idu/hugegraph/schema/builder/EdgeLabelBuilder.java	86.69% <100%> (+0.06%)	58 <3> (ø)	⬇️
... and 1 more

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