check baseUrl is OAI-PMH target before harvesting

Check by carrying out an Identify request, raise an appropriate
Exception if not.

oaiharvest/harvest.py

@@ -56,6 +56,7 @@
 from oaipmh.client import Client
 from oaipmh.error import NoRecordsMatchError
 
+from .exceptions import NotOAIPMHBaseURLException
 from .metadata import DefaultingMetadataRegistry, XMLMetadataReader
 from .registry import verify_database
 
@@ -75,6 +76,14 @@ def _listRecords(self, baseUrl, metadataPrefix="oai_dc", **kwargs):
         # Add metatdataPrefix to args
         kwargs['metadataPrefix'] = metadataPrefix
         client = Client(baseUrl, metadata_registry)
+        # Check that baseUrl actually represents an OAI-PMH target
+        try:
+            client.identify()
+        except IndexError:
+            raise NotOAIPMHBaseURLException(
+                "{0} does not appear to be an OAI-PMH compatible base URL"
+                "".format(baseUrl)
+            )
         # Check server timestamp granularity support
         client.updateGranularity()
         for record in client.listRecords(**kwargs):