make metadata exceptions optional based on CLI flag

make metadata exceptions optional based on CLI flag

In many cases, there are times where metadata errors need to be ignored, or
at least not stop the command from running.  For example, there will
inevitably be new metadata fields added, in which case a packaged version
of fdroidserver will throw errors on each one.  This adds a standard -W
flag to customize the response: ignore, default, or error.

* by default, the errors are still errors
* `fdroid readmeta -W` will just print errors
* `fdroid readmeta -Wignore` will not even print errors

https://gitlab.com/fdroid/fdroidserver/issues/150