Update gradle wrapper information
Define IsDirectoryException

Basically, an equivalent to POSIX' EISDIR. Strangely enough, this is not defined
by java.nio.file, and a bug has been found insofar as that as of this writing,
at least on Unix-like systems, trying and opening an InputStream on a directory
does not fail; it only fails when attempting to _read_ from it (and it fails
with the message matching errno's EISDIR).