commands: add request parser error variant for keyword without value

src/commands.rs

@@ -612,6 +612,12 @@ pub enum RequestParserError {
         found: u32,
     },
 
+    #[error("A keyword argument was provided without a value: {keyword}")]
+    MissingKeywordValue {
+        /// The unexpected keyword that was found
+        keyword: String,
+    },
+
     #[error("A command list was expected to be closed, but the end was not found")]
     MissingCommandListEnd,