Also interpret lines starting with "!!" as errors from the firmware

(see http://reprap.org/wiki/G-code#Replies_from_the_RepRap_machine_to_the_host_computer)

CHANGELOG.md

@@ -41,6 +41,7 @@
   * The "Slicing done" notification is now colored green ([#558](https://github.com/foosel/OctoPrint/issues/558)).
 * File management now supports STL files as first class citizens (including UI adjustments to allow management of
   uploaded STL files including removal and reslicing) and also allows folders (not yet supported by UI)
+* Also interpret lines starting with "!!" as errors
 
 ### Bug Fixes
 

src/octoprint/util/comm.py

@@ -986,7 +986,7 @@ class MachineCom(object):
 
 	def _handleErrors(self, line):
 		# No matter the state, if we see an error, goto the error state and store the error for reference.
-		if line.startswith('Error:'):
+		if line.startswith('Error:') or line.startswith('!!'):
 			#Oh YEAH, consistency.
 			# Marlin reports an MIN/MAX temp error as "Error:x\n: Extruder switched off. MAXTEMP triggered !\n"
 			#	But a bed temp error is reported as "Error: Temperature heated bed switched off. MAXTEMP triggered !!"