Refactor server error handling

[zhulik]

grumlin/lib/grumlin/request_error_factory.rb

Lines 17 to 27 in e99d101

	# Neptune presumably returns message as a JSON string of format
	# {"detailedMessage":"",
	# "requestId":"UUID",
	# "code":"ConcurrentModificationException"}
	# Currencly we simply search for substings to identify the exact error
	# TODO: parse json and use `code` instead
	VERTEX_ALREADY_EXISTS = "Vertex with id already exists:"
	EDGE_ALREADY_EXISTS = "Edge with id already exists:"
	CONCURRENT_VERTEX_INSERT_FAILED = "Failed to complete Insert operation for a Vertex due to conflicting concurrent"
	CONCURRENT_EDGE_INSERT_FAILED = "Failed to complete Insert operation for an Edge due to conflicting concurrent"
	CONCURRENCT_MODIFICATION_FAILED = "Failed to complete operation due to conflicting concurrent"