[r=jameinel] environs/imagemetadat: tweak debug messages
[r=jameinel] environs/imagemetadat: tweak debug messages

We were generating messages when we failed to read an index file, but
didn't have one when we succeeded. Which meant it looks like things
were failing when really we were just moving to a fallback.

Also, we had a formatting error "%q/%q" causes each part of the URL to
get its own " characters. We only want the whole string quoted.

I extracted out the base+path code into urlJoin, because I didn't see
something obvious elswhere in the code, I just have it as a local
function. We really should put this as a general helper, though.

https://codereview.appspot.com/12834045/
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