Add throwOnError setting

Added new setting throwOnError which will have the client rethrow any
exception it encountered when calling Send(). This can be used for
debugging or if you wish to run the client in a fail fast manner.
Fix for NullReferenceException issue

Refactored the method a bit to tidy it up while resolving this.
Merge pull request #62 from MindscapeHQ/throw-on-error

Add throwOnError setting
Merge pull request #64 from MindscapeHQ/fix-issue-63

Fix for NullReferenceException issue
Merge branch 'master' into fix-issue-65

Conflicts:
	Mindscape.Raygun4Net/Messages/RaygunRequestMessage.cs