46cd4174

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Fix watch crash on attachment in a nonexistent directory

When an attachment's path points into a directory that does not exist (for
example because of a typo), checkAttachment tried to list that parent directory
with listDirRel to suggest similar filenames, which threw an uncaught IO
exception (getDirectoryContents:openDirStream: does not exist). The watch loop
only catches ExitCode, so this crashed the whole watch process.

Wrap the directory listing in forgivingAbsence so a nonexistent parent
directory simply yields no suggestions, and the normal CE_MISSING_ATTACHMENT
error is reported instead.

Adds a regression test scenario (CE_MISSING_ATTACHMENT_NONEXISTENT_DIR) that
crashed before the fix.