Say what a site sent when it is not XML at all

doghouse answers 200 with "Unable to establish a DB connection", and parse()
reported two parser errors about end of input that buried it. A body that does
not start with < now reports its first line, and explain_failure flags it as
the publisher's problem. Malformed XML keeps the parsers' errors.

Fixes #2

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SZbKERNSt4vQfyGV8rvkqp

CHANGELOG.md

@@ -10,6 +10,12 @@ The long form, with what was wrong before and how it was found, is in
 
 ## [Unreleased]
 
+### Changed
+
+- A feed whose site sends a message instead of the feed, such as "Unable to establish a DB
+  connection", now shows that message and is flagged as the publisher's problem, instead of two
+  parser errors about reaching the end of input.
+
 ### Removed
 
 - An unused icon glyph (`minus`) left over from before Unsubscribe settled on `circleMinus`.