link_outputs: don't fail if output file doesn't exist

[mlin]

Currently link_outputs fails (errors out) if a File output isn't an extant, local filename. Normally it should be; but plugins may want to rewrite task outputs for various reasons (e.g. to uploaded S3 URIs). So relax link_outputs to just skip & warn any non-extant outputs. (But we'd still like some protection against inadvertent bugs outputting non-existent files in plain-vanilla operations...)