[word_parse.py] mention compat reason in backtick errors

doc/error-catalog.md

@@ -272,6 +272,22 @@ Try one of these alternatives:
 
     ysh$ echo $['--flag=' ++ myvar]  # expression sub
 
+
+### OILS-ERR-18
+
+```
+  echo "date = `date`"
+               ^
+[ -c flag ]:1: Backtick should be $(cmd) or \` (parse_backticks, OILS-ERR-18)
+```
+
+- Did you mean to use `$(date)` instead?
+  - This is the only syntax for command substitution in YSH, because it nests
+    cleanly.
+- Did you mean to escape the backtick with `` \` ``?
+  - (OSH and YSH have the same string literal syntax, including the
+    backslash-quoted backtick `` \` ``.)
+
 ## Runtime Errors - Traditional Shell
 
 These errors may occur in shells like [bash]($xref) and [zsh]($xref).

osh/word_parse.py

@@ -1095,8 +1095,7 @@ def _ReadLikeDQ(self, left_token, is_ysh_expr, out_parts):
 
             elif self.token_kind == Kind.Left:
                 if self.token_type == Id.Left_Backtick and is_ysh_expr:
-                    p_die("Invalid backtick: use $(cmd) or \\` in YSH strings",
-                          self.cur_token)
+                    p_die("Backtick should be $(cmd) or \\` (OILS-ERR-18)", self.cur_token)
 
                 part = self._ReadDoubleQuotedLeftParts()
                 out_parts.append(part)
@@ -1210,7 +1209,7 @@ def _ReadCommandSub(self, left_id, d_quoted=False):
 
         elif left_id == Id.Left_Backtick:
             if not self.parse_opts.parse_backticks():
-                p_die('Use $(cmd) instead of backticks (parse_backticks)',
+                p_die('Backtick should be $(cmd) or \\` (parse_backticks, OILS-ERR-18)',
                       left_token)
 
             self._SetNext(lex_mode_e.Backtick)  # advance past `