better jeepney error text (#188)

If dbus-based method is failing because DBUS_SESSION_BUS_ADDRESS is not set, give better error message

Author: fohrloop

wakepy/dbus_adapters/jeepney.py

@@ -5,6 +5,10 @@
 from wakepy.core import DbusAdapter, DbusMethodCall
 
 
+class DbusNotFoundError(RuntimeError):
+    ...
+
+
 class JeepneyDbusAdapter(DbusAdapter):
     """An implementation of DbusAdapter using jeepney. Can be used to process
     DbusMethodCalls (communication with Dbus services over a dbus-daemon)."""
@@ -25,7 +29,18 @@ def process(self, call: DbusMethodCall):
             signature=call.method.signature,
             body=call.args,
         )
-        connection = open_dbus_connection(bus=call.method.bus)
+        try:
+            connection = open_dbus_connection(bus=call.method.bus)
+        except KeyError as exc:
+            if "DBUS_SESSION_BUS_ADDRESS" in str(exc):
+                raise DbusNotFoundError(
+                    "The environment variable DBUS_SESSION_BUS_ADDRESS is not set! "
+                    "To use dbus-based methods with jeepney, a session (not system) "
+                    "bus (dbus-daemon process) must be running, and the address of the "
+                    "bus should be available at the DBUS_SESSION_BUS_ADDRESS "
+                    "environment variable. To check if you're running a session "
+                    "dbus-daemon, run `ps -x | grep dbus-daemon`"
+                ) from exc
         reply = connection.send_and_get_reply(msg, timeout=self.timeout)
         resp = unwrap_msg(reply)
         return resp