docs: add a clear message uppon ban

Author: ManonMarchand

CHANGES.rst

@@ -62,6 +62,8 @@ xmatch
 - the API is more flexible: you can now ommit the ``vizier:`` before the catalog name
   when crossmatching with a vizier table [#3194]
 
+- add a help message when people are banned instead of returning error code 403 [#3225]
+
 Infrastructure, Utility and Other Changes and Additions
 -------------------------------------------------------
 

astroquery/xmatch/core.py

@@ -111,6 +111,14 @@ def query_async(self, cat1, cat2, max_distance, *, colRA1=None, colDec1=None,
 
         response = self._request(method='POST', url=self.URL, data=payload,
                                  timeout=self.TIMEOUT, cache=cache, **kwargs)
+
+        if response.status_code == 403:
+            raise HTTPError("Your IP address has been banned from the XMatch server. "
+                            "This means that you sent too many cross-matching jobs in "
+                            "parallel to the service blocking other astronomers. Please"
+                            " contact the CDS team at cds-question[at]unistra.fr to "
+                            "find a solution.")
+
         try:
             response.raise_for_status()
         except HTTPError as err:

docs/xmatch/xmatch.rst

@@ -79,6 +79,17 @@ in the resulting table for demonstration purposes.  Finally, ``colRa1`` and
 Troubleshooting
 ===============
 
+403 Forbidden
+-------------
+
+If you are getting a 403 Forbidden error, then your IP address has been banned from
+XMatch's server. This means that you sent too many cross-matching jobs in parallel to 
+the service blocking other astronomers. Please contact the CDS team at
+cds-question[at]unistra.fr to find a solution.
+
+Out of date results
+-------------------
+
 If you are repeatedly getting failed queries, or bad/out-of-date results, try clearing your cache:
 
 .. code-block:: python