Improve slightly the error message for files not found.

Before:
  Unable to load file '//test:b.bzl': file doesn't exist or isn't a file

After:
  Unable to load file '//test:b.bzl': file doesn't exist

#6004

RELNOTES: None.
PiperOrigin-RevId: 217512790
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/ASTFileLookupValue.java b/src/main/java/com/google/devtools/build/lib/skyframe/ASTFileLookupValue.java
index e42ba4e..7f38607 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/ASTFileLookupValue.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/ASTFileLookupValue.java
@@ -101,9 +101,14 @@
         String.format("Unable to load package for '%s': %s", fileLabel, reason));
   }
 
+  static ASTFileLookupValue forMissingFile(Label fileLabel) {
+    return new ASTLookupNoFile(
+        String.format("Unable to load file '%s': file doesn't exist", fileLabel));
+  }
+
   static ASTFileLookupValue forBadFile(Label fileLabel) {
     return new ASTLookupNoFile(
-        String.format("Unable to load file '%s': file doesn't exist or isn't a file", fileLabel));
+        String.format("Unable to load file '%s': it isn't a regular file", fileLabel));
   }
 
   public static ASTFileLookupValue withFile(BuildFileAST ast) {