frontend: Fix plugin manager config loading crash

Current code isn't catching a parse error exception if an invalid config
file is loaded by the plugin manager. This change wraps the plugin
manager config json parse call with a try/catch, and handles invalid
config files by creating a new config file, and logs an error that the
existing config file is invalid.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f