patch 8.2.1444: error messages are spread out and names can be confusing

Problem:    Error messages are spread out and names can be confusing.
Solution:   Start moving error messages to a separate file and use clear
            names.
diff --git a/src/evalvars.c b/src/evalvars.c
index 70afa84..43c05fd 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -805,7 +805,7 @@
 						   || !IS_WHITE_OR_NUL(*expr)))
 	    {
 		vim_strncpy(op, expr - len, len);
-		semsg(_(e_white_both), op);
+		semsg(_(e_white_space_required_before_and_after), op);
 		i = FAIL;
 	    }