fix: The backend store of 'xx' has not been initialized

[javeme]

The backend has been initialized, but this error is still raised.

fix: #632
Change-Id: I9890559365c01bd73bc85c25e1cb058c2637fb0d

[codecov]

Codecov Report

Merging #708 into master will decrease coverage by 0.24%.
The diff coverage is 40.8%.

@@             Coverage Diff              @@
##             master     #708      +/-   ##
============================================
- Coverage     71.41%   71.16%   -0.25%     
- Complexity     4328     4330       +2     
============================================
  Files           280      281       +1     
  Lines         20593    20667      +74     
  Branches       2893     2909      +16     
============================================
+ Hits          14706    14708       +2     
- Misses         4444     4515      +71     
- Partials       1443     1444       +1

Impacted Files	Coverage Δ	Complexity Δ
...om/baidu/hugegraph/backend/query/QueryResults.java	87.14% <ø> (ø)	33 <0> (ø)	⬇️
...u/hugegraph/traversal/optimize/HugeVertexStep.java	72.46% <ø> (ø)	18 <0> (ø)	⬇️
...om/baidu/hugegraph/backend/store/BackendStore.java	87.5% <ø> (ø)	4 <0> (ø)	⬇️
...gegraph/backend/store/rocksdb/RocksDBSessions.java	100% <ø> (ø)	1 <0> (ø)	⬇️
...baidu/hugegraph/plugin/HugeGraphGremlinPlugin.java	85.71% <ø> (ø)	4 <0> (ø)	⬇️
...raph/backend/store/rocksdb/RocksDBStdSessions.java	69.72% <0%> (+0.07%)	29 <0> (ø)	⬇️
...c/main/java/com/baidu/hugegraph/cmd/InitStore.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...du/hugegraph/backend/cache/CachedBackendStore.java	9.67% <0%> (-0.33%)	0 <0> (ø)
...h/backend/store/rocksdbsst/RocksDBSstSessions.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...h/backend/store/postgresql/PostgresqlSessions.java	66.66% <0%> (-3.61%)	8 <0> (ø)
... and 24 more

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[javeme]

this BackendException | IllegalStateException | UncheckedIOException catch are for rocksdb, cassandra, hbase fetch without database&table