[GR-14735] Do not suppress the stacktrace of internal exceptions

* Handle the -S case more simply. TopLevelRaiseHandler takes care of all
  RaiseException raised during `Truffle::Boot.main`.
* PolyglotException#printStackTrace() also shows a nicely-formatted
  stacktrace, including guest-languages frames.

Author: eregon

src/launcher/java/org/truffleruby/launcher/RubyLauncher.java

@@ -16,6 +16,7 @@
 import org.graalvm.polyglot.Engine;
 import org.graalvm.polyglot.PolyglotException;
 import org.graalvm.polyglot.Source;
+import org.graalvm.polyglot.Value;
 import org.truffleruby.shared.options.OptionsCatalog;
 import org.truffleruby.shared.TruffleRuby;
 import org.truffleruby.shared.Metrics;
@@ -209,7 +210,13 @@ private int runRubyMain(Context.Builder contextBuilder, CommandLineOptions confi
                         TruffleRuby.BOOT_SOURCE_NAME).internal(true).buildLiteral();
 
                 config.executionAction = ExecutionAction.FILE;
-                config.toExecute = context.eval(source).execute(config.toExecute).asString();
+                final Value file = context.eval(source).execute(config.toExecute);
+                if (file.isString()) {
+                    config.toExecute = file.asString();
+                } else {
+                    System.err.println("truffleruby: No such file or directory -- " + config.toExecute + " (LoadError)");
+                    return 1;
+                }
             }
 
             final Source source = Source.newBuilder(TruffleRuby.LANGUAGE_ID,
@@ -222,7 +229,7 @@ private int runRubyMain(Context.Builder contextBuilder, CommandLineOptions confi
             Metrics.printTime("after-run");
             return exitCode;
         } catch (PolyglotException e) {
-            System.err.println(TruffleRuby.SIMPLE_NAME + ": " + e.getMessage());
+            e.printStackTrace();
             return 1;
         }
     }

src/main/ruby/truffleruby/core/truffle/boot.rb

@@ -44,8 +44,8 @@ def self.find_s_file(name)
     path = find_in_environment_paths(name, ENV['PATH']) and return path
     return name if File.exist?(name)
 
-    # Fail otherwise
-    raise LoadError, "No such file or directory -- #{name}"
+    # Not found, let the RubyLauncher print the error
+    nil
   end
 
   def self.find_in_environment_paths(name, env_value)