Xunit1 exception handling - report thrown from class start/finish & handle nested exceptions

Adds support for exceptions thrown in class start/finish in xunit1. Currently, exceptions thrown from fixture constructors or Dispose methods are silently ignored, and the test appears to succeed.

Also adds support for nested exceptions in xunit1. Currently, nested exceptions are "mangled" into a single set of exception details. This change parses the mangled strings and reports exceptions as separate.