Fixes a regression introduced in
d27cc3bfaafe6b5b0831e88afb1c46311d382a0b.
Reported by Ricardo Wurmus <rekado@elephly.net>.
* guix/discovery.scm (scheme-files): Catch 'scandir*' system errors.
Return '() and optionally raise a warning upon 'system-error'.
* tests/discovery.scm ("scheme-modules, non-existent directory"): New
test.