Display compiler's error location in stacktrace.

`clojure.lang.Compiler$CompilerException`s carry the reader location of
the error, but not much else. Display the compiler exception's
location description (when present); then show the details from its
original cause.
I'm not sure if `class` could ever be `nil`; if so - using `string=` might not be a good idea as it will blow up for a `nil` argument.