This commit broke the initialisation of the union backend
f17d7c0012 union: refactor to use fspath.SplitFs instead of fs.ParseRemote #4996
This patch fixes it.
This adds a context.Context parameter to NewFs and related calls.
This is necessary as part of reading config from the context -
backends need to be able to read the global config.
Before this change we errored out if one upstream errored in Purge or
About.
This change checks for fs.ErrorDirNotFound and skips that backend in
this case.