Check SqlState instead of error message

The PostgreSQL error messages are translated, so trying to match them against an english text might not work on different locales.

src/Weasel.Postgresql/Tables/Table.FetchExisting.cs

@@ -205,7 +205,7 @@ private async Task readPartitionsAsync(DbDataReader reader, Table existing, Canc
         }
         catch (PostgresException e)
         {
-            if (e.Message.Contains("does not exist")) return;
+            if (e.SqlState == "3F000") return; // 3F000 = ERRCODE_INVALID_SCHEMA_NAME
             throw;
         }