Fixes 2 issues related to loading plugins

1. Fixes an issue where the error message "Incompatible Cluster: mycluster" would be printed when a plugin fails initialization. Now prints "ERROR - An error occurred while loading plugins: {back trace} Failed to load plugin {xyz} with the following error: {err_msg}. This error may occur if the plugin code is modified after the cluster was created.

2. Fixes an issue where the error message "ERROR - cluster template smallcluster does not exist" when trying to launch a cluster with plugins that fail initialization. Now prints an error message similar than the one from point 1.