Add standard error to exception message

The rationale is that when castxml has an error it raises a RuntimeError to the user with the message.
However, the message only contains the stdout from castxml - this makes the RuntimeError contain no information about what actually went wrong.
This change allows the user to add error handling to their applications based on what went wrong with castxml (such as changing their include directories, etc)

pygccxml/parser/source_reader.py

@@ -235,7 +235,8 @@ def create_xml_file(self, source_file, destination=None):
         process = subprocess.Popen(
             args=command_line,
             shell=True,
-            stdout=subprocess.PIPE)
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE)
 
         try:
             results = []
@@ -246,9 +247,12 @@ def create_xml_file(self, source_file, destination=None):
             for line in process.stdout.readlines():
                 if line.strip():
                     results.append(line.rstrip())
+            for line in process.stderr.readlines():
+                if line.strip():
+                    results.append(line.rstrip())
 
             exit_status = process.returncode
-            msg = os.linesep.join([str(s) for s in results])
+            msg = os.linesep.join([str(s.decode()) for s in results])
             if self.__config.ignore_gccxml_output:
                 if not os.path.isfile(xml_file):
                     raise RuntimeError(

unittests/source_reader_tester.py

@@ -4,6 +4,7 @@
 # See http://www.boost.org/LICENSE_1_0.txt
 
 import unittest
+import os
 
 from . import parser_test_case
 
@@ -31,6 +32,17 @@ def test_compound_argument_type(self):
         self.assertTrue(do_smth, "unable to find do_smth")
         do_smth.function_type()
 
+    def test_stderr_present_and_readable(self):
+        with open(os.path.join('unittests', 'data', self.header), 'r') as f:
+            source_str = f.read()
+
+        err_str = "add some stuff that should not compile"
+        source_str += err_str
+        with self.assertRaises(RuntimeError) as e_context:
+            decls = parser.parse_string(source_str, self.config)
+
+        self.assertIn(err_str, e_context.exception.args[0])
+
 
 def create_suite():
     suite = unittest.TestSuite()