NFC: Refactor a string so we can add a comment to it.

Test: Run cargo2android.py
Change-Id: Id833bd3292a4cd9f8c348aee9725e0924d826d97
diff --git a/scripts/cargo2android.py b/scripts/cargo2android.py
index 786cf85..c5585f3 100755
--- a/scripts/cargo2android.py
+++ b/scripts/cargo2android.py
@@ -95,6 +95,9 @@
 
 CARGO_OUT = 'cargo.out'  # Name of file to keep cargo build -v output.
 
+# This should be kept in sync with tools/external_updater/crates_updater.py.
+ERRORS_LINE = 'Errors in ' + CARGO_OUT + ':'
+
 TARGET_TMP = 'target.tmp'  # Name of temporary output directory.
 
 # Message to be displayed when this script is called without the --run flag.
@@ -1355,7 +1358,7 @@
         if self.args.dependencies and self.dependencies:
           self.dump_dependencies()
         if self.errors:
-          self.append_to_bp('\nErrors in ' + CARGO_OUT + ':\n' + self.errors)
+          self.append_to_bp('\n' + ERRORS_LINE + '\n' + self.errors)
     return self
 
   def add_ar_object(self, obj):