Better error handling for GenerateMainDexList

Unwrap ExecutionException, and report all errors on the provided
DiagnosticsHandler same way as the D8, R8 and DexSplitter.

Bug: 112679440
Change-Id: I6b803f0e6585c1461eeacfcac1845b363508ef69
diff --git a/src/main/java/com/android/tools/r8/GenerateMainDexListCommand.java b/src/main/java/com/android/tools/r8/GenerateMainDexListCommand.java
index f9e0ebf..2d06da8 100644
--- a/src/main/java/com/android/tools/r8/GenerateMainDexListCommand.java
+++ b/src/main/java/com/android/tools/r8/GenerateMainDexListCommand.java
@@ -150,6 +150,10 @@
     return mainDexListConsumer;
   }
 
+  Reporter getReporter() {
+    return reporter;
+  }
+
   private static void parse(String[] args, GenerateMainDexListCommand.Builder builder) {
     for (int i = 0; i < args.length; i++) {
       String arg = args[i].trim();