Allow to suppress warnings for already removed files

[eliasp]

Affects fclones 0.24.0:

I ran group on a huge collection of various files and then started to remove individual sub-trees of them, as I wanted to focus on a specific type/set of files on each iteration.

Once I continued, fclones remove --dry-run … started spamming me with warnings regarding the already removed files:

[2022-05-24 23:43:18.996] fclones:  warn: Failed to read metadata of '/some/path/to/removed/file.1': No such file or directory (os error 2)
[2022-05-24 23:43:18.996] fclones:  warn: Failed to read metadata of '/some/path/to/removed/file.2': No such file or directory (os error 2)
[2022-05-24 23:43:18.996] fclones:  warn: Could not determine files to drop in group with hash 1ef18153318010dc5b4b9f7834d722f1 and len 89: Metadata of some files could not be obtained

I'd like to be able to control this behavior, e.g. by using the global --quiet flag to suppress those warnings.