SI-4492 More informative error when class not found on classpath

Position the error based on Select tree that failed to type check,
presumably due to an underlying MissingRequirementError, which has no position.

There are lots of other ways we could rewrap a MRE and supplement position info,
but that remains TODO. Jason's review comment is recorded in the code.

Also try to detect the case of a missing module and provide some advice,
as well as linking to the forthcoming 2.11 guide at
http://docs.scala-lang.org/overviews/core/scala-2.11.html.
Merge pull request #3631 from adriaanm/t4492

SI-4492 More informative error when class not found on classpath
SI-8425 don't create double-dollar names in c.freshName

If we append a dollar to a user-provided prefix that ends in a dollar,
we create a potential for confusion for backend phases. That's why
this commit prevents such situations from happening.
Merge pull request #3638 from xeno-by/topic/freshname-hotfix

SI-8425 don't create double-dollar names in c.freshName