Fix NullPointerException and 'no more capacity' issues in blob store

[jsjtzyy]

1. NPE occurs when error message of captured IOException is null. In
   this case, we should use Objects.equals to compare error string.
2. 'no more capacity' issue occurs when new log segment creation failed,
   however, the code didn't restore the counter of remainingUnallocatedSegments.
   This patch will catch such failure and restore counter as well as clean
   up the allocated file.

This pr will address #1167

[zzmao]

Thanks for fixing it!

[codecov-io]

Codecov Report

Merging #1168 into master will decrease coverage by 0.13%.
The diff coverage is 29.72%.

@@             Coverage Diff              @@
##             master    #1168      +/-   ##
============================================
- Coverage     70.03%   69.89%   -0.14%     
+ Complexity     5352     5347       -5     
============================================
  Files           426      426              
  Lines         32650    32654       +4     
  Branches       4142     4143       +1     
============================================
- Hits          22865    22825      -40     
- Misses         8653     8686      +33     
- Partials       1132     1143      +11

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/com.github.ambry.store/HardDeleter.java	81.58% <0%> (+0.14%)	42 <0> (ø)	⬇️
...c/main/java/com.github.ambry.store/LogSegment.java	86.66% <0%> (+1.42%)	50 <0> (ø)	⬇️
...n/java/com.github.ambry.store/PersistentIndex.java	92.37% <100%> (+0.26%)	197 <0> (+1)	⬆️
...tore/src/main/java/com.github.ambry.store/Log.java	93.1% <27.77%> (-5.33%)	60 <0> (ø)
...main/java/com.github.ambry.store/IndexSegment.java	86.1% <42.85%> (+0.14%)	106 <0> (+1)	⬆️
...c/main/java/com.github.ambry.network/Selector.java	77.22% <0%> (-3.48%)	47% <0%> (-1%)
...java/com.github.ambry.store/CompactionManager.java	87.33% <0%> (-2.67%)	19% <0%> (ø)
...github.ambry.rest/AsyncRequestResponseHandler.java	88.59% <0%> (-2.29%)	23% <0%> (ø)
...in/java/com.github.ambry.store/BlobStoreStats.java	71.13% <0%> (-2.27%)	103% <0%> (-2%)
.../main/java/com.github.ambry.store/ScanResults.java	81.25% <0%> (-1.57%)	16% <0%> (-1%)
... and 6 more

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[cgtz]

The logic for going from IOException to StoreErrorCode is repeated in a lot of places. Since this is a bugfix, you don't have to address it now, but it would be good to move this into a static method in StoreException in the future.