NullPointerException when error message of captured exception is null

[jsjtzyy]

#1025 is merged with aim to catch disk I/O error. However, comparing error message with IO_ERROR_STR defined in StoreException is not a good way because error message could be null for IOException. Following is two examples from server log:

2019/05/03 00:00:00.130 ERROR [ReplicaThread] [Replica Thread-Inter0**-***] [ambry-server] [] Remote node: DataNode[***-****.***.linkedin.com:****] Thread name: Replica Thread-Inter0**-*** Remote replica: Replica[***-****.***.linkedin.com:****:/mnt/u010/ambrydata/43]
java.lang.NullPointerException

2019/05/04 00:11:34.476 ERROR [DiskManager] [store-startup-Partition[165]] [ambry-server] [] Exception while starting store for the partitionPartition[165]
com.github.ambry.store.StoreException: Error while starting store for dir /mnt/u010/ambrydata/165
        at com.github.ambry.store.BlobStore.start(BlobStore.java:233)
        at com.github.ambry.store.DiskManager.lambda$start$17(DiskManager.java:125)
        at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException
        at com.github.ambry.store.LogSegment.<init>(LogSegment.java:139)
        at com.github.ambry.store.Log.getSegmentsToLoad(Log.java:342)
        at com.github.ambry.store.Log.<init>(Log.java:78)
        at com.github.ambry.store.BlobStore.start(BlobStore.java:209)
        ... 2 more

We should first address NPE problem in this case and then figure out a good way to determine if IOException is related to Disk issue.

[jsjtzyy]

addressed in #1168