docs: add unit in error message

Author: ManonMarchand

astroquery/xmatch/core.py

@@ -94,7 +94,7 @@ def query_async(self, cat1, cat2, max_distance, *, colRA1=None, colDec1=None,
             The HTTP response returned from the service.
         """
         if max_distance > 180 * u.arcsec:
-            raise ValueError('max_distance argument must not be greater than 180')
+            raise ValueError('max_distance argument must not be greater than 180".')
         payload = {'request': 'xmatch',
                    'distMaxArcsec': max_distance.to(u.arcsec).value,
                    'RESPONSEFORMAT': 'votable',

astroquery/xmatch/tests/test_xmatch.py

@@ -46,10 +46,9 @@ def request_mockreturn(method, url, data, **kwargs):
 
 
 def test_xmatch_query_invalid_max_distance():
-    with pytest.raises(ValueError) as ex:
+    with pytest.raises(ValueError,
+                       match='max_distance argument must not be greater than 180"'):
         XMatch().query_async('', '', 181 * arcsec)
-        assert str(ex.value) == (
-            'max_distance argument must not be greater than 180')
 
 
 def test_get_available_tables(monkeypatch):