Just: refuse a path that is not there rather than formatting nothing

Naming a file is an assertion that it exists, and a typo in one was answered
with the silence that means everything already matched, exit 0 included, under a
banner claiming the formatter had run on it. Only the named path is checked: a
directory holding nothing to format is a fair answer, and so is a glob whose
expansion this formatter does not claim.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: redsun82

misc/just/run_on_files.py

@@ -7,7 +7,9 @@
 
 The command is run once per batch of file names rather than once per file, and the
 batches are sized so that no single command line runs into a length limit. Nothing is
-run at all when no file matches.
+run at all when no file matches, so silence means that nothing here matched rather than
+that nothing was there: a path that does not exist is refused instead, naming one being
+an assertion that it does.
 """
 
 import argparse
@@ -160,6 +162,9 @@ def parse_args():
     args.command, args.paths = args.rest[:separator], args.rest[separator + 1 :]
     if not args.command:
         parser.error("no command given")
+    missing = [path for path in args.paths if not os.path.exists(path)]
+    if missing:
+        parser.error("no such path: " + ", ".join(missing))
     return args