Let LUTFinder discover LUTs inside .jar files again

The optimization to use toURI().getPath() unfortunately broke handling
jar:file:... URLs completely.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Let LUTFinder throw a more helpful exception

The message itself is only half as useful if not accompanied by the
stack trace.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
LUTFinder: use the context class loader

If we really limit the LUTFinder to the class loader that loaded
the class, we can neither add more .jar files later nor test
efficiently. Therefore: don't.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Make sure that LUTFinder handles spaces in file names correctly

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'lut-finder'

This topic branch -- together with the fix in scijava-common's 1.8.1
release -- helps the problem where the lookup tables in "NCSA PalEdit"
were not discovered correctly due to the space in the directory's
file name.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>