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/errors.py

@@ -0,0 +1,9 @@
+"""
+Custom exception and error classes for Nominatim.
+"""
+
+class UsageError(Exception):
+    """ An error raised because of bad user input. This error will usually
+        not cause a stack trace to be printed unless debugging is enabled.
+    """
+    pass