introduce custom UsageError

This is a exception to be thrown when the error occures because
of bad user data. We don't want to print a full stack trace in
these cases but just tell the user what went wrong.

nominatim/db/connection.py

@@ -27,7 +27,7 @@ class _Cursor(psycopg2.extras.DictCursor):
         self.execute(sql, args)
 
         if self.rowcount != 1:
-            raise ValueError("Query did not return a single row.")
+            raise RuntimeError("Query did not return a single row.")
 
         return self.fetchone()[0]
 

nominatim/db/status.py

@@ -6,6 +6,7 @@ import logging
 import re
 
 from ..tools.exec_utils import get_url
+from ..errors import UsageError
 
 LOG = logging.getLogger()
 
@@ -19,7 +20,7 @@ def compute_database_date(conn):
 
         if osmid is None:
             LOG.fatal("No data found in the database.")
-            raise RuntimeError("No data found in the database.")
+            raise UsageError("No data found in the database.")
 
     LOG.info("Using node id %d for timestamp lookup", osmid)
     # Get the node from the API to find the timestamp when it was created.
@@ -31,7 +32,7 @@ def compute_database_date(conn):
     if match is None:
         LOG.fatal("The node data downloaded from the API does not contain valid data.\n"
                   "URL used: %s", node_url)
-        raise RuntimeError("Bad API data.")
+        raise UsageError("Bad API data.")
 
     LOG.debug("Found timestamp %s", match[1])