Improve plugin loading error message

When a plugin doesn't have the right type (i.e. plugin name in the plugin module isn't of type Plugin, see plugins02 test case), we get a quite terse error message:

The value plugin did not have the type Plugin as required

In a complex build it is not very useful. We would like to get more information:

1. the name of the module containing the bogus plugin name
2. the current type of the plugin name
3. slightly enhance the error message to give a qualified name for the expected Plugin type (i.e. GHC.Plugins.Plugin)