Storage API must work with exceptions, not booleans

[PVince81]

Currently the Storage API is similar to the one from PHP and simply returns false when an error occurred.
This makes it difficult to distinguish whether "false" means "File not found" or "Storage not available".

We should move to using exceptions in such cases and get rid of booleans.

@Xenopathic @icewind1991 @schiesbn @DeepDiver1975

[RobinMcCorkell]

I think it comes down to catching 3rd party exceptions then wrapping them into our exceptions. Perhaps some system for a storage to define what exceptions may be thrown from it, and what they should 'map' to? For example, the DAV storage backend would map Sabre\DAV\Exception\NotFound to OCP\Files\NotFoundException. Then, without any extra try catch blocks in the storages themselves, a wrapper or the filesystem itself would catch such errors, then wrap them in our exceptions.

try {
    $storage->doSomething();
} catch (\Exception $e) {
    foreach ($storage->getExceptionMap() as $orig => $wrapper) {
        if (is_instance($e, $orig)) {
            throw new $wrapper(..., $e);
        }
    }
    throw new \OCP\Files\UnknownFailureException(..., $e);
}

Only issue I can see is one of performance...

[PVince81]

If we consider that exceptions do not happen often, then performance wouldn't be much of an issue.