Abort startup on a config file pixa cannot read (closes #176)

Of the places pixa looks for its config file on its own, it passed over
any place where os.Stat failed, so a file in a directory pixa may not
enter was skipped without a word and pixa started on a later file or on
the environment and defaults. Now only a file that does not exist is
passed over; any other error aborts startup naming the file, as a file
that does not parse already did. README.md says so where it gives the
search order.

Model: opus-5-5

TODO.md

@@ -29,6 +29,11 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-10-04 a config file pixa cannot read aborts startup (closes #176): of the
+  places pixa looks for its config file on its own, only one where the file does
+  not exist is passed over; any other error, such as a directory on the path
+  that pixa may not enter, aborts startup naming the file, as a file that does
+  not parse already did.
 - 2026-10-04 the metrics basic auth, CORS preflight, request logging and
   metrics recording have tests (closes #79): `MetricsAuth` on its own answers
   401 with a challenge without credentials or with a wrong username or password