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 files that were meant to be excluded 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
