Mention all the requirements for MiniRacer on TruffleRuby (#257)

Raise there instead of waiting for the `@context.eval` below, for a clearer error.

lib/mini_racer/truffleruby.rb

@@ -71,7 +71,9 @@ module MiniRacer
       end
 
       unless Polyglot.languages.include? "js"
-        warn "You also need to install the 'js' component with 'gu install js' on GraalVM 22.2+", uplevel: 0 if $VERBOSE
+        raise "The language 'js' is not available, you likely need to `export TRUFFLERUBYOPT='--jvm --polyglot'`\n" \
+          "You also need to install the 'js' component with 'gu install js' on GraalVM 22.2+\n" \
+          "Note that you need TruffleRuby+GraalVM and not just the TruffleRuby standalone to use MiniRacer"
       end
 
       @context = Polyglot::InnerContext.new(on_cancelled: -> {