aconfig: update error message in modify_parsed_flags_based_on_mode

This commit updates the error message in
modify_parsed_flags_based_on_mode, when there is no certain mode flags
in the corresponding library, and this change remove "." at the end of
the error message to conform to existing standard in this project.

Bug: 311152507
Test: atest aconfig.test
Change-Id: Ib120e7f7252981bbeb6ca1cf785da24f329507b4
diff --git a/tools/aconfig/src/commands.rs b/tools/aconfig/src/commands.rs
index cd53371..0c4f543 100644
--- a/tools/aconfig/src/commands.rs
+++ b/tools/aconfig/src/commands.rs
@@ -339,7 +339,7 @@
         }
     };
     if modified_parsed_flags.is_empty() {
-        bail!("{} library contains no exported flags.", codegen_mode);
+        bail!("{codegen_mode} library contains no {codegen_mode} flags");
     }
 
     Ok(modified_parsed_flags)
@@ -653,9 +653,6 @@
         parsed_flags.parsed_flag.retain_mut(|pf| !pf.is_exported());
         let error =
             modify_parsed_flags_based_on_mode(parsed_flags, CodegenMode::Exported).unwrap_err();
-        assert_eq!(
-            format!("{} library contains no exported flags.", CodegenMode::Exported),
-            format!("{:?}", error)
-        );
+        assert_eq!("exported library contains no exported flags", format!("{:?}", error));
     }
 }