The changes done with cee46311e84ba41e8a421b47099ad709e23761f1 cause an
exception when an URL is passed as argument, which contains improperly
escaped characters (most noticeably whitespaces) and therefore causes an
URISyntaxException when calling URL.toURI().

This does not occur when using IPath.fromOSString(), which is the
approach that was done previously. But instead of converting this path
back to an OS-dependent string, it is instead converted to a file, to
check whether it exists and to only then convert it back to its String
representation.

A test case with an ill-formed URL has been added to avoid a similar
problem in the future.