Improve handling of invalid userpaths. [1674]

This looks for three potential problems with the userpath:

* /path/name/here is a file rather than a directory.

* No read access to /path/name/here.

* No write access to /path/name/here.

These situations will result in something like the following getting
printed to stderr (remember, the logging code won't have access to the
userpath at this point...so no mcidasv.log!):

McIDAS-V could not initialize itself. Please report this message and
the following output to mug@ssec.wisc.edu.

java.lang.IllegalArgumentException: '/path/name/here' must be both
readable and writable by McIDAS-V.
<followed by a stack trace>