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Write a more correct error message.

src/hooks.rs

@@ -103,7 +103,7 @@ impl Hook {
                                             )
                                         })?,
                                     )?,
-                                    None => bail!("Missing expression in variable `{}`", token),
+                                    None => bail!("Invalid expression `{}`", token),
                                 };
 
                                 command.push_str(&replaced);