Narrow causes in NoSuchThingException family

Become a bit more restrictive to keep our exception cases tight.

--
MOS_MIGRATED_REVID=107953779
diff --git a/src/main/java/com/google/devtools/build/lib/packages/BuildFileNotFoundException.java b/src/main/java/com/google/devtools/build/lib/packages/BuildFileNotFoundException.java
index 4eed155..ca4ffed 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/BuildFileNotFoundException.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/BuildFileNotFoundException.java
@@ -16,6 +16,8 @@
 
 import com.google.devtools.build.lib.cmdline.PackageIdentifier;
 
+import java.io.IOException;
+
 /**
  *  Exception indicating an attempt to access a package which is not found or
  *  does not exist.
@@ -27,7 +29,7 @@
   }
 
   public BuildFileNotFoundException(PackageIdentifier packageIdentifier, String message,
-      Throwable cause) {
+      IOException cause) {
     super(packageIdentifier, message, cause);
   }
 }