Avoid hiding exceptions when trying to format the stack trace

Hi,

while using lein cljs-build to compile ClojureScript code, I have been annoyed several times by ClojureScript
compiler syntax errors not being reported by clj-stacktrace. Some syntax errors made the stack trace trimming  throw itself an exception hiding the compiler exception.

This made the auto mode of cljs-build crash since the exception was thrown at the plugin itself.

I tested cljs-build with the latest release of clj-stacktrace (0.2.8) w/o improvements, hence this patch.

It avoids the whole issue by trapping the trimming error and returning the exception as is.
Better having a raw output than hiding it... It was invaluable in nailing down syntax errors.
My personal code parser is not always acute :)

I did not see any open issue for this, cljs-build has one, if you can push this fix out, I'll add comments
on the cljs-build issue to try to get a new release out.

Thank you,
Luc P.