Better db_sqlite errors when db_sqlite not connected.

lib/impure/db_sqlite.nim

@@ -126,6 +126,7 @@ proc tryExec*(db: DbConn, query: SqlQuery,
               args: varargs[string, `$`]): bool {.
               tags: [ReadDbEffect, WriteDbEffect].} =
   ## tries to execute the query and returns true if successful, false otherwise.
+  assert(not db.isNil, "Database not connected.")
   var q = dbFormat(query, args)
   var stmt: sqlite3.Pstmt
   if prepare_v2(db, q, q.len.cint, stmt, nil) == SQLITE_OK:
@@ -144,6 +145,7 @@ proc newRow(L: int): Row =
 
 proc setupQuery(db: DbConn, query: SqlQuery,
                 args: varargs[string]): Pstmt =
+  assert(not db.isNil, "Database not connected.")
   var q = dbFormat(query, args)
   if prepare_v2(db, q, q.len.cint, result, nil) != SQLITE_OK: dbError(db)
 
@@ -267,6 +269,7 @@ proc tryInsertID*(db: DbConn, query: SqlQuery,
                   {.tags: [WriteDbEffect], raises: [].} =
   ## executes the query (typically "INSERT") and returns the
   ## generated ID for the row or -1 in case of an error.
+  assert(not db.isNil, "Database not connected.")
   var q = dbFormat(query, args)
   var stmt: sqlite3.Pstmt
   result = -1