Backtrace: add first line (origin of Exception)

Before, the backtrace only captured and formatted the return value of
->getTrace().  According to a comment in the docs:

From: http://www.php.net/manual/en/exception.gettrace.php#107563
 > Two important points about this function which are not documented:
 >
 > 1) [->getTrace()] does not include the file / line at which the
 >    exception is thrown; that entry is only recorded in the top-level
 >    getFile/Line methods.

This change ensures that the file and line the exception was actually
created on are passed through as part of the backtrace.