(PUP-2631) Elide long URIs for 404 message

When the agent encounters a 404 response from the master and it has been
told to fail on 404, it previously output the entire path plus query
string that it requested. However, the most common case where this
occurs is when requesting a catalog, which can have a very long query
string if the request was done with a GET. This does not always occur
because even longer query strings will result in the agent using a POST
instead of a GET, which causes the long query string output problem to
go away.

In order to fix this the query string is now elided to 100 characters
(97 characters of the original text plus 3 periods).  In order to still
be able to debug any problems that might result from the query, however,
the query string is *not* elided if the log level is `debug`.
Merge pull request #2971 from zaphod42/issue/master/pup-2631-non-existant-env-produces-long-message

(PUP-2631) Elide long URIs for 404 message