Capture non-existent block within a scope, and report a nice error message

[fsaad]

ripl.execute_program('assume x1 = tag("x", "x", normal(0,1))')
ripl.execute_program('assume y1 = tag("y", "y", normal(x1,1))')
ripl.observe('y1', 2)
ripl.execute_program('infer(mh("x", 1, 100))')

The final line crashes noisily since block 1 does not exist in scope "x", leading this dictionary lookup in line 344 to fail:

Venturecxx/backend/lite/trace.py

Lines 341 to 346 in 9304f69

	def getNodesInBlock(self, scope, block, do_copy=True):
	#import pdb; pdb.set_trace()
	#scope, block = self._normalizeEvaluatedScopeAndBlock(scope, block)
	nodes = self.scopes[scope][block]
	# Defensively copying the set of nodes, because
	# unregisterRandomChoice will mutate them.

[fsaad]

Remember to handle the built-in scopes/blocks such as "all", "one", etc.