Parse <message> node correctly

Message can be reported this way:

 <message>
   <message_level val="info"/>
   <string>mypred_Sx&nbsp;is&nbsp;defined</string>
 </message>

When recving message like this, python raise an exception
in get_answer() because accessing 'c[2]' is invalid in this case.

We should look for the exact node instead.

Signed-off-by: Wang Nan <wangnan0@huawei.com>

autoload/coqtop.py

@@ -173,6 +173,14 @@ def escape(cmd):
               .replace("(", '(') \
               .replace(")", ')')
 
+# Extract <string> from a <message> element
+def parse_message(elt):
+    for e in elt:
+        if e.tag == 'string':
+            return parse_value(e)
+    # message without string?
+    return ''
+
 def get_answer():
     fd = coqtop.stdout.fileno()
     data = ''
@@ -192,9 +200,9 @@ def get_answer():
                         valueNode = c
                     if c.tag == 'message':
                         if messageNode is not None:
-                            messageNode = messageNode + "\n\n" + parse_value(c[2])
+                            messageNode = messageNode + "\n\n" + parse_message(c)
                         else:
-                            messageNode = parse_value(c[2])
+                            messageNode = parse_message(c)
                 if shouldWait:
                     continue
                 else: