doc comments on MetaValError, remove unused variant

Author: AsbjornOlling

llama-cpp-2/src/lib.rs

@@ -82,17 +82,18 @@ pub enum ChatTemplateError {
     Utf8Error(#[from] std::str::Utf8Error),
 }
 
+/// Failed fetching metadata value
 #[derive(Debug, Eq, PartialEq, thiserror::Error)]
 pub enum MetaValError {
-    #[error("model does not have metadata key: {0}")]
-    MissingKey(String),
-
+    /// The provided string contains an unexpected null-byte
     #[error("null byte in string {0}")]
     NullError(#[from] NulError),
 
+    /// The returned data contains invalid UTF8 data
     #[error("FromUtf8Error {0}")]
     FromUtf8Error(#[from] FromUtf8Error),
 
+    /// Got negative return value. This happens if the key or index queried does not exist.
     #[error("Negative return value. Likely due to a missing index or key. Got return value: {0}")]
     NegativeReturn(i32),
 }