Improve exception handling

Adds a generic error handler for unsupported databases, and makes the error messages for ipv4 and ipv6 exceptions more verbose and less stilted. Response to #68.

pygeoip/__init__.py

@@ -408,11 +408,13 @@ class GeoIP(object):
 
         :arg addr: IPv4 or IPv6 address (eg. 203.0.113.30)
         """
+        if self._databaseType in (const.PROXY_EDITION, const.NETSPEED_EDITION_REV1, const.NETSPEED_EDITION_REV1_V6):
+            raise GeoIPError('Invalid database type; this database is not supported')
         ipv = 6 if addr.find(':') >= 0 else 4
         if ipv == 4 and self._databaseType not in (const.COUNTRY_EDITION, const.NETSPEED_EDITION):
-            raise GeoIPError('Invalid database type; expected IPv6 address')
+            raise GeoIPError('Invalid database type; this database supports IPv6 addresses, not IPv4')
         if ipv == 6 and self._databaseType != const.COUNTRY_EDITION_V6:
-            raise GeoIPError('Invalid database type; expected IPv4 address')
+            raise GeoIPError('Invalid database type; this database supports IPv4 addresses, not IPv6')
 
         ipnum = util.ip2long(addr)
         return self._seek_country(ipnum) - const.COUNTRY_BEGIN