fix: render error messages for invalid templates (#3088)

Currently direct templates can be created without the required
signatures fields for signers.

This means that the document can be fully signed by everyone but will
ultimately fail the sealing step which leaves the document in an
unrecoverable state.

packages/lib/server-only/document/send-document.ts

@@ -194,7 +194,7 @@ export const sendDocument = async ({ id, userId, teamId, sendEmail, requestMetad
       .map((r) => (r.name ? `${r.name} (${r.email}, id: ${r.id})` : `${r.email} (id: ${r.id})`))
       .join(', ');
 
-    throw new AppError(AppErrorCode.INVALID_REQUEST, {
+    throw new AppError(AppErrorCode.MISSING_SIGNATURE_FIELD, {
       message: `The following recipients are missing required fields: ${missingRecipientDescriptions}. Signers must have at least one signature field.`,
     });
   }