Elements rename JavaScript exceptions occurring during action-sends. (fixes #63)

So that `SLTest` will attach call-site information recorded by the `UIAElement`
macro to those exceptions.
Only format call site information into error messages if we have call site information.

We would not have call site information if an `SLUIAElement` threw an exception
without the test writer having used the `UIAElement` macro.
Prevent recorded call site information from being reused between test cases.

So that, at the least, it won't be reused between test cases
(though we can't guarantee it won't be reused within a test case,
between a successful `SLUIAElement` call where the test writer used the `UIAElement` macro
and an unsuccessful `SLUIAElement` call where the test writer didn't use the macro).
Test that call site information recorded by the `UIAElement` macro will be attached
to `SLUIAElement` exceptions.
Merge pull request #64 from inkling/jeff/63_tag_JS_exceptions_with_callsite_info

Elements rename JavaScript exceptions occurring during action-sends. (fi...