fix: a type, plus add the type-check to the mobile build scripts

src/libs/endorserServer.ts

@@ -1686,7 +1686,10 @@ export async function register(
         "Registration thrown error:",
         errorMessage || JSON.stringify(err),
       );
-      return { error: errorMessage || "Got a server error when registering." };
+      return {
+        error:
+          (errorMessage as string) || "Got a server error when registering.",
+      };
     }
     return { error: "Got a server error when registering." };
   }