JarUtil: Use common strings for exceptions (same as used in IOUtil)
IOUtil/JarUtil: DEBUG output on stderr not stdout
IOUtil: Add comments to slashify args @ caller; Use validated File @ IOUtil.toURISimple ; DEBUG: stdout -> stderr

Add comments to slashify(..) arguments for better documentation.

+++

Use validated File @ IOUtil.toURISimple(..):
    slashify(file, true, isDirectory)
to
    slashify(new File(path).getAbsolutePath(), true, isDirectory)

i.e. same w/ above variant and determine absolute path via File instance.

+++

DEBUG: stdout -> stderr
JARUtil/IOUtil: Reuse IOUtil.JAR_SCHEME_SEPARATOR (now a char); Add DEBUG to JarUtil.getJarEntry(..)
IOUtil.toURL(..) Apply decodeFromURI(uri.getPath()) if file-scheme; Add IOUtil.decodeURIToFilePath(..) for native usage.

Refine comments, API doc.

toURL(..): Apply space conversion, decodeFromURI(..), on file-scheme path,
           ensuring decoded space.

++

Add decodeURIToFilePath(String uriPath) and decodeURIToFilePath(URI uri)

Both methods shall simplify decoding a file-URI for native platform usage.

Tested in TestIOUtilURIHandling

+++
IOUtil.encodeToURI(): Only use method if required

Only use IOUtil.encodeToURI() if required,
i.e. 'new URI(String)' but not 'new URI(scheme, ...)' since the latter already encodes the path.

TestIOUtilURIHandling.test00BasicCoding() validates above findings.