Making FileNotFoundException open to allow subclassing it

[nathanfallet]

In some cases, we would like to subclass FileNotFoundException (the same way it subclasses IOException for example). But it was currently not possible since it was not open.

Example from kdriver:

class BrowserExecutableNotFoundException : FileNotFoundException(
    """
    Could not determine browser executable.
    Make sure your browser is installed in the default location (path).
    Or specify browserExecutablePath parameter.
    """.trimIndent()
)

This is okay on JVM, but does not work on other platforms yet (until this PR gets merged and released 😎)