Custom try-catch block for jinja

Allows some more error checking in templates. Not used at the moment but
might come in handy in the future.

src/octoprint/server/__init__.py

@@ -623,6 +623,7 @@ class Server():
 
 	def _setup_jinja2(self):
 		app.jinja_env.add_extension("jinja2.ext.do")
+		app.jinja_env.add_extension("octoprint.server.util.jinja.trycatch")
 
 		# configure additional template folders for jinja2
 		import jinja2

src/octoprint/server/util/jinja.py

@@ -0,0 +1,60 @@
+# coding=utf-8
+from __future__ import absolute_import
+
+__author__ = "Gina Häußge <osd@foosel.net>"
+__license__ = 'GNU Affero General Public License http://www.gnu.org/licenses/agpl.html'
+__copyright__ = "Copyright (C) 2015 The OctoPrint Project - Released under terms of the AGPLv3 License"
+
+from jinja2 import nodes
+from jinja2.ext import Extension
+
+import logging
+
+class ExceptionHandlerExtension(Extension):
+	tags = {"try"}
+
+	def __init__(self, environment):
+		super(ExceptionHandlerExtension, self).__init__(environment)
+		self._logger = logging.getLogger(__name__)
+
+	def parse(self, parser):
+		token = parser.stream.next()
+		lineno = token.lineno
+		filename = parser.name
+		error = parser.parse_expression()
+
+		args = [error, nodes.Const(filename), nodes.Const(lineno)]
+		try:
+			body = parser.parse_statements(["name:endtry"], drop_needle=True)
+			node = nodes.CallBlock(self.call_method("_handle_body", args),
+			                       [], [], body).set_lineno(lineno)
+		except Exception as e:
+			# that was expected
+			self._logger.exception("Caught exception while parsing template")
+			node = nodes.CallBlock(self.call_method("_handle_error", [nodes.Const(self._format_error(error, e, filename, lineno))]),
+			                       [], [], []).set_lineno(lineno)
+
+		return node
+
+	def _handle_body(self, error, filename, lineno, caller):
+		try:
+			return caller()
+		except Exception as e:
+			self._logger.exception("Caught exception while compiling template {filename} at line {lineno}".format(**locals()))
+			error_string = self._format_error(error, e, filename, lineno)
+			return error_string if error_string else ""
+
+	def _handle_error(self, error, caller):
+		return error if error else ""
+
+	def _format_error(self, error, exception, filename, lineno):
+		if not error:
+			return ""
+
+		try:
+			return error.format(exception=exception, filename=filename, lineno=lineno)
+		except:
+			self._logger.exception("Error while compiling exception output for template {filename} at line {lineno}".format(**locals()))
+			return "Unknown error"
+
+trycatch = ExceptionHandlerExtension