Use notifications for errors

[ntotten]

Currently, fatal errors like if we cannot load the prettier module, issues with the config file, problems loading modules, etc. are printed to the output and the status bar icon changes in some cases, but otherwise it is very hard to know an error happened. We should use VSCode's notifications for errors and warnings that are fatal - we dont want to be too aggressive (i think for formatting errors due to invalid code on the file being formatted for example, a notification is too aggressive - for that we should use problems, but that is a separate issue.

Update the code to show actionable and useful error or warning notifications on errors. For example if we cannot load the prettier module because it isn't installed locally, we should notify the user of the error and tell them that we found prettier in the package.json, but it is not installed and they need to install it.