Error thrown when EmitDocumentation is true and no XML-file is present

[enkelmedia]

At the moment, if the "EmitDocumentation"-flag is true in the option but there is no .xml-file with documentation for the assembly the TypeScriptGenerator will thrown an exception.

After thinking about this, that might be a good thing to throw and say that one need to activate xml-documentation for the project?

Reproduce:

• Right click the Test-project and choose "Properties"
• Go to the "Build" tab in the properties window
• Uncheck "XML documentation file" and Save
• Clean the output-directory and run the test-project.

Given that we want to throw the exception, I guess that the "EmitDocumentation"-flag should default to false otherwise this might cause issue for someone new to the project that don't understand what this flag does. It would also be a breaking change to default this flag to true.