Improve error message for unsupported named parameters:

Previously just 'Illegal parameter number' was logged, make this a more
helpful message.

```
[25-05-06 20:46:04.2629] Slim::Schema::Storage::throw_exception (122) Error: DBI Exception: DBD::mysql::st bind_param failed: named parameters are unsupported: :album [for Statement "
                                        SELECT contributor_album.role AS role, contributors.name AS name, contributors.id AS id
                                        FROM contributor_album
                                        JOIN contributors ON contributors.id = contributor_album.contributor
                                        WHERE contributor_album.album = :album
                                        AND contributor_album.role IN (5,1)
                                        ORDER BY contributor_album.role, contributors.namesort
                                "]
```

Author: JKDingwall

dbdimp.c

@@ -4207,6 +4207,23 @@ int dbd_bind_ph(SV *sth, imp_sth_t *imp_sth, SV *param, SV *value,
   attribs= attribs;
   maxlen= maxlen;
 
+  if (!looks_like_number(param))
+  {
+    STRLEN len;
+    char *paramstring;
+    paramstring = SvPV(param, len);
+    if(paramstring[len] == 0 && strlen(paramstring) == len)
+    {
+      do_error(sth, JW_ERR_ILLEGAL_PARAM_NUM, form("named parameters are unsupported: %s", paramstring), NULL);
+      return FALSE;
+    }
+    else
+    {
+      do_error(sth, JW_ERR_ILLEGAL_PARAM_NUM, "<param> could not be coerced to a C string", NULL);
+      return FALSE;
+    }
+  }
+
   if (param_num <= 0  ||  param_num > DBIc_NUM_PARAMS(imp_sth))
   {
     do_error(sth, JW_ERR_ILLEGAL_PARAM_NUM, "Illegal parameter number", NULL);