sourcemaps upload: Warn when no minified JS files would be uploaded #3248
Description
Somewhat contrary to its name, the sourcemaps upload command is responsible for uploading not only sourcemaps, but also the corresponding minified source files. Without the minified files, symbolication doesn't work; in other words, a sourcemap uploaded without its minified file does nothing. sentry-cli sourcemaps upload should at least warn and possibly even error if no minified files would be uploaded, since there is no scenario in which this is useful and it indicates a probable user error.
For an example of how this might happen, say you call sentry-cli sourcemaps upload --ext jssm ./dist because for whatever reason your sourcemaps don't end in .js.map but .jssm. By passing --ext you have cleared the default list of extensions and replaced it with just jssm. This means that minified files (usually ending in .js, .mjs, …) won't be uploaded. The correct thing to do in this scenario is to pass the --ext option multiple times, once for each relevant extension.