Previously, (read-partition-uuid "/does/not/exist") would return #f.
With this change, a 'system-error exception is raised as expected.
* gnu/build/file-systems.scm (ENOENT-safe): Clarify docstring.
(partition-field-reader): Remove use of 'ENOENT-safe'.
(partition-predicate): Wrap READER in 'ENOENT-safe'.