[geary/wip/conversation-polish: 1/22] Fix uncaught error build warning

[Michael Gratton <mjog src gnome org>]

commit a4af3a0b55a70c4e6fd345ef76f78e96497b9aa6
Author: Michael Gratton <mike vee net>
Date:   Wed Jan 23 15:51:59 2019 +1100

    Fix uncaught error build warning

 src/engine/imap-db/imap-db-gc.vala | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/src/engine/imap-db/imap-db-gc.vala b/src/engine/imap-db/imap-db-gc.vala
index dd170850..183bb2f8 100644
--- a/src/engine/imap-db/imap-db-gc.vala
+++ b/src/engine/imap-db/imap-db-gc.vala
@@ -618,7 +618,8 @@ private class Geary.ImapDB.GC {
         return deleted;
     }
 
-    private async bool has_message_rows(GLib.Cancellable? cancellable) {
+    private async bool has_message_rows(GLib.Cancellable? cancellable)
+        throws GLib.Error {
         bool ret = false;
         yield db.exec_transaction_async(Db.TransactionType.RO, (cx) => {
                 Db.Result result = cx.query(