Enhancement: Validate exclude patterns before backing up

Exclude patterns provided via `--exclude`, `--iexclude`, `--exclude-file` or `--iexclude-file`
previously weren't validated. As a consequence, invalid patterns resulted in all files being backed up.
restic now validates all patterns before running the backup and aborts with a fatal error
if an invalid pattern is detected.

https://github.com/restic/restic/issues/3709
https://github.com/restic/restic/pull/3734
