Skip invalid plugins entries for help and plugin commands

[GrahamCampbell]

Since #395, an invalid plugins entry in serverless.yml fails initialization before any command runs. That is the right default, but it also affects the two contexts that were already lenient about plugins that are listed in the configuration without being installed: printing help and running the plugin management commands. In particular, a broken entry such as serverless-webpack@1.2.3 made it impossible to run serverless plugin uninstall, because the command failed on the very configuration it is meant to fix, leaving hand-editing the file as the only way out.

This change reuses the existing leniency rule instead of introducing a new one. parsePluginsObject gains a tolerateInvalidEntries option that drops invalid entries with a warning instead of throwing, and resolveServicePlugins enables it under exactly the condition that already skips plugins failing with PLUGIN_NOT_FOUND: when help is requested or when the current command is help or plugin. Everywhere else the validation continues to fail hard, the packaging code path keeps the strict default, and the upgrade guide now mentions the softer behavior in its section on plugins validation.