patch 8.2.3069: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move some error messages to errors.h.  Use clearer names.
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index d8c5a12..7e0bd64 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -2120,7 +2120,7 @@
 	    // with UNIX it is possible to open a directory
 	    if (mch_isdir(ffname))
 	    {
-		semsg(_(e_isadir2), ffname);
+		semsg(_(e_src_is_directory), ffname);
 		return FAIL;
 	    }
 #endif
@@ -2137,7 +2137,7 @@
 	    else
 #endif
 	    {
-		emsg(_(e_exists));
+		emsg(_(e_file_exists));
 		return FAIL;
 	    }
 	}
@@ -3586,7 +3586,7 @@
     if (secure)
     {
 	secure = 2;
-	emsg(_(e_curdir));
+	emsg(_(e_command_not_allowed_from_vimrc_in_current_dir_or_tag_search));
 	return TRUE;
     }
 #ifdef HAVE_SANDBOX
@@ -3729,7 +3729,7 @@
 	    ++cmd;
 	    if (vim_strchr((char_u *)"/?&", *cmd) == NULL)
 	    {
-		emsg(_(e_backslash));
+		emsg(_(e_backslash_should_be_followed_by));
 		return;
 	    }
 	    if (*cmd != '&')
@@ -3944,7 +3944,7 @@
     if (!subflags.do_count && !curbuf->b_p_ma)
     {
 	// Substitution is not allowed in non-'modifiable' buffer
-	emsg(_(e_modifiable));
+	emsg(_(e_cannot_make_changes_modifiable_is_off));
 	return;
     }
 
@@ -4903,7 +4903,7 @@
 	++cmd;
 	if (vim_strchr((char_u *)"/?&", *cmd) == NULL)
 	{
-	    emsg(_(e_backslash));
+	    emsg(_(e_backslash_should_be_followed_by));
 	    return;
 	}
 	if (*cmd == '&')