NEXUS-5612: File listing should fail in case of IO problem

Currently DefaultFSPeer was hiding the null returned
from File.listFiles. According to javadocs, it returns null
in two cases, a) when the File instance does not denote
a directory, or b) in case of IO error.

As case a) is checked few lines above (if check failed,
we would not get here), the null return value should
be treated as IO Error.

By throwing a generic LocalStorageExcetion, we
make this case to be treated as error.
Merge remote-tracking branch 'origin/master' into file-list-io-error
Merge remote-tracking branch 'origin/master' into file-list-io-error
Merge remote-tracking branch 'origin/master' into file-list-io-error
NEXUS-5612: Testing interaction with WalkerProcessor and FSPeer

Especially the case when FSPeer#listItems method throws
LocalStorageException.

The Walk must be stopped, after this, no processor
should have onCollectionExit invoked.

This UT actually simulates the case when due to
too many open files, FSPeer made Walker think
that collection is empty, while it was not.

DefaultFSPeer is already fixed in prev commit,
this UT only ensures that Walker, Context and
Processors behave correctly in this case.
NEXUS-5612: Remove UT since tag

Remnant from copied skeleton UT.
Merge pull request #791 from sonatype/file-list-io-error

[2.5] NEXUS-5612 File listing should fail in case of IOEx