When cargo errors, print an error about cargo.out.

Currently if cargo produces an error it's not obvious how to debug it.
Add a message telling the user to look at the cargo.out file, which
has its output.

Test: Ran on crates with and without errors
Change-Id: Ib681a8e87f60467c7425aed5f7e5277a0603bcc8

scripts/cargo2android.py

@@ -1280,7 +1280,10 @@ class Runner(object):
           print('Running:', cmd)
         with open(cargo_out, 'a') as out_file:
           out_file.write('### Running: ' + cmd + '\n')
-        os.system(cmd)
+        ret = os.system(cmd)
+        if ret != 0:
+          print('*** There was an error while running cargo.  ' +
+                'See the cargo.out file for details.')
     if added_workspace:  # restore original Cargo.toml
       with open(cargo_toml, 'w') as out_file:
         out_file.writelines(cargo_toml_lines)