(core) show any error when creating initial team site with GRIST_SINGLE_ORG

Summary: Also clarifies that only lowercase letters are accepted.

Test Plan: manual

Reviewers: jarek

Reviewed By: jarek

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D3647

app/common/orgNameUtils.ts

@@ -41,7 +41,7 @@ const BLACKLISTED_SUBDOMAINS = new Set([
 export function checkSubdomainValidity(subdomain: string): void {
   // stick with limited alphanumeric subdomains.
   if (!(/^[a-z0-9][-a-z0-9]*$/.test(subdomain))) {
-    throw new Error('Domain must include letters, numbers, and dashes only.');
+    throw new Error('Domain must include lower-case letters, numbers, and dashes only.');
   }
   // 'docs-*' is reserved for personal orgs.
   if (subdomain.startsWith('docs-')) { throw new Error('Domain cannot use reserved prefix "docs-".'); }