Error messages for a couple other boundary conditions

src/reflog.c

@@ -483,8 +483,10 @@ int git_reflog_drop(
 
 	entry = (git_reflog_entry *)git_reflog_entry_byindex(reflog, idx);
 
-	if (entry == NULL)
+	if (entry == NULL) {
+		giterr_set(GITERR_REFERENCE, "No reflog entry at index "PRIuZ, idx);
 		return GIT_ENOTFOUND;
+	}
 
 	reflog_entry_free(entry);
 

src/refs.c

@@ -844,8 +844,10 @@ static int reference__update_terminal(
 	git_reference *ref;
 	int error = 0;
 
-	if (nesting > MAX_NESTING_LEVEL)
+	if (nesting > MAX_NESTING_LEVEL) {
+		giterr_set(GITERR_REFERENCE, "Reference chain too deep (%d)", nesting);
 		return GIT_ENOTFOUND;
+	}
 
 	error = git_reference_lookup(&ref, repo, ref_name);