Allow certain failures for 'from' part of rewrite

[jsha]

Right now the checker fetches both the HTTP version and the rewritten HTTPS version of a URL, and fails if either one fails.

There are a small number of cases in which we don't mind if the HTTP version fails. Specifically, a connection failure or timeout on the HTTP version are more or less okay. These failures prevent us from measuring a delta between the HTTP and HTTPS versions, but as long as the rule is simple (rewrite everything on HTTP to the same path on HTTPS), it's okay to assume resources are equivalent.

[jsha]

Note: Ideally we would allow this type of failure only for simple targets (i.e. those specified using the <simpletarget ... /> tag), but that is not yet implemented.