Improve failure message of java/nio/file/WatchService/SensitivityModifier.java

The failure message observed in the parent issue does not convey much information. Specifically there is nothing about the sensitivity setting for the parent directory of the file for which there was a failure. It would be useful to add this information. For example, if the failure is always at LOW sensitivity, then some adjustment in the test might be necessary. This might indicate the failure arises from slowness in the polling implementation on macOS.