API: use default's APIError exception instead of the VaultAPIError

this later does not brings anything over the default's one.

This requires swh.core >= 0.0.56

requirements-swh.txt

-swh.core >= 0.0.44
+swh.core >= 0.0.56
 swh.model >= 0.0.27
 swh.objstorage >= 0.0.17
 swh.scheduler >= 0.0.39

swh/vault/api/client.py

@@ -7,20 +7,9 @@ from swh.model import hashutil
 from swh.core.api import SWHRemoteAPI
 
 
-class VaultAPIError(Exception):
-    """Vault API Error"""
-    def __str__(self):
-        return ('An unexpected error occurred in the Vault backend: {}'
-                .format(self.args))
-
-
 class RemoteVaultClient(SWHRemoteAPI):
     """Client to the Software Heritage vault cache."""
 
-    def __init__(self, url, timeout=None):
-        super().__init__(
-            api_exception=VaultAPIError, url=url, timeout=timeout)
-
     # Web API endpoints
 
     def fetch(self, obj_type, obj_id):