ManTweet install fails at step 5

[dregad]

APPLICATION ERROR #2503
Upgrading the plugin schema failed in block #5.

This is on a fresh MantisBT install with latest master as of this writing (mantisbt/mantisbt@a21eb8c)

[dregad]

I wonder if this upgrade step has ever worked...

Upgrading the plugin with $g_display_errors[E_ALL] = 'inline'; shows the following error
SYSTEM WARNING: 'call_user_func() expects parameter 1 to be a valid callback, function 'install_install_mantweet_purge_cached_entries' not found or invalid function name' in '/home/dregad/dev/mantisbt/core/plugin_api.php' line 763

UpdateFunction argument (the function name) must not include the install_ prefix, as it is added by the API

Also, successful return value for install steps should be 2 (that's how ADOdb works), even though using true work since we're using a loose comparison (PHP casts the expression to boolean).