OSLExpressionEngine : Fix handling of runtime errors from OSL

[johnhaddon]

Description

When OSL encounters a runtime error, it reports this via the ErrorHandler given to the ShadingSystem. Because we are not providing an error handler, the default is to print an error to the terminal but otherwise continue. Instead, we need to catch the error and turn it into an exception that halts Gaffer's current computation.

Steps to reproduce

This unit test shows the problem.

	def testExpressionError( self ) :

		s = Gaffer.ScriptNode()

		s["n"] = Gaffer.Node()
		s["n"]["user"]["o"] = Gaffer.IntPlug( flags = Gaffer.Plug.Flags.Default | Gaffer.Plug.Flags.Dynamic )

		s["e"] = Gaffer.Expression()
		s["e"].setExpression( 'string i[2] = { "one", "two" }; parent.n.user.o = strlen( i[2] );', "OSL" )

		with self.assertRaisesRegex( Gaffer.ProcessException, r".*Index \[2\] out of range.*" ) :
			s["n"]["user"]["o"].getValue()

Implementation notes

OSL's ErrorHandler class isn't given any access to per-shade state, and we use a single ShadingSystem (and therefore ErrorHandler) for all OSL expressions. We'll need to create an error handler that delegates to a per-thread stack of handlers that we can push and pop to direct the errors appropriately. Something like this :

shadingSystem = ShadingSystem( ThreadLocalErrorHandler() )

CapturingErrorHandler myHandler;
{
    ThreadLocalErrorHandler::HandlerScope myScope( myHandler );
    shadingSystem->execute();
}

if( myHandler.errors().size() )
{
    throw ...
}