Make Synthetic name error message more explicit

Change-Id: I4979a6e0a9d54f6f3a24b58a60c4ff1f182a98ac
diff --git a/src/main/java/com/android/tools/r8/synthesis/SyntheticNaming.java b/src/main/java/com/android/tools/r8/synthesis/SyntheticNaming.java
index 37e327f..e2a6f66 100644
--- a/src/main/java/com/android/tools/r8/synthesis/SyntheticNaming.java
+++ b/src/main/java/com/android/tools/r8/synthesis/SyntheticNaming.java
@@ -321,7 +321,7 @@
         binaryName.lastIndexOf(
             kind.isFixedSuffixSynthetic() ? kind.descriptor : SYNTHETIC_CLASS_SEPARATOR);
     if (index < 0) {
-      throw new Unreachable("Unexpected failure to compute an synthetic prefix");
+      throw new Unreachable("Unexpected failure to compute a synthetic prefix for " + binaryName);
     }
     return binaryName.substring(0, index);
   }