Mention cljc when require fails

If you attempt to require a namespace for which the code cannot be located, now that cljc files are considered when locating code, it could be mentioned in the list of files considered on the classpath.

Note: FWIW, ClojureScript has similar error messages, and the order listed in the error message is the order tried when loading. If this were followed, I suspect the text of the error message above would end up looking like:

Patch: clj-1797.patch
Approach: add info about cljcfile not found in exception thrown from load
Screened by: Alex Miller