Try to improve error reporting when streaming a PCAP file. Ref #2235.

gns3server/controller/compute.py

@@ -348,6 +348,12 @@ class Compute:
         response = yield from self._session().request("GET", url, auth=self._auth, timeout=None)
         if response.status == 404:
             raise aiohttp.web.HTTPNotFound(text="{} not found on compute".format(path))
+        elif response.status == 403:
+            raise aiohttp.web.HTTPForbidden(text="forbidden to open {} on compute".format(path))
+        elif response.status != 200:
+            raise aiohttp.web.HTTPInternalServerError(text="Unexpected error {}: {}: while opening {} on compute".format(response.status,
+                                                                                                                         response.reason,
+                                                                                                                         path))
         return StreamResponse(response)
 
     @asyncio.coroutine