Ignore invalid characters when reading the output of a process

It should happend only when user try to use another binary and
the code of VPCS, dynamips... Will detect it's not the
correct binary.

For example we detect this error after an user used SupperPutty
instead of VPCS.

Fix #235

gns3server/utils/asyncio.py

@@ -52,7 +52,10 @@ def subprocess_check_output(*args, cwd=None, env=None):
     output = yield from proc.stdout.read()
     if output is None:
         return ""
-    return output.decode("utf-8")
+    # If we received garbage we ignore invalid characters
+    # it should happend only when user try to use another binary
+    # and the code of VPCS, dynamips... Will detect it's not the correct binary
+    return output.decode("utf-8", errors="ignore")
 
 
 @asyncio.coroutine