Improve a few error messages

Change-Id: I7725f67cd7779c46b128981bf6b7bb86c36fe8b7
diff --git a/src/main/java/com/android/tools/r8/D8Command.java b/src/main/java/com/android/tools/r8/D8Command.java
index 766371d..46f1ce4 100644
--- a/src/main/java/com/android/tools/r8/D8Command.java
+++ b/src/main/java/com/android/tools/r8/D8Command.java
@@ -460,7 +460,9 @@
           reporter.error(
               "D8 does not support main-dex inputs and outputs when compiling to API level "
                   + AndroidApiLevel.L_MR1.getLevel()
-                  + " and above");
+                  + " and above (min API level "
+                  + getMinApiLevel()
+                  + " was provided)");
         }
       }
       if (hasDesugaredLibraryConfiguration() && getDisableDesugaring()) {
@@ -479,7 +481,9 @@
           reporter.error(
               "D8 startup layout requires native multi dex support (API level "
                   + AndroidApiLevel.L.getLevel()
-                  + " and above)");
+                  + " and above, min API level "
+                  + getMinApiLevel()
+                  + " was provided)");
         }
       }
       super.validate();