Fix handling of sqlite out of memory errors

The "error_code == -1" check never passed, as error_code (confusingly) is a
string of a number.

qutebrowser/misc/sql.py

@@ -35,6 +35,7 @@ class SqliteErrorCode:
     in qutebrowser here.
     """
 
+    UNKNOWN = '-1'
     BUSY = '5'  # database is locked
     READONLY = '8'  # attempt to write a readonly database
     IOERR = '10'  # disk I/O error
@@ -114,7 +115,8 @@ def raise_sqlite_error(msg, error):
     ]
 
     if (error_code in environmental_errors or
-            (error_code == -1 and database_text in environmental_strings)):
+            (error_code == SqliteErrorCode.UNKNOWN and
+             database_text in environmental_strings)):
         raise SqlEnvironmentError(msg, error)
     else:
         raise SqlBugError(msg, error)

tests/unit/misc/test_sql.py

@@ -49,6 +49,15 @@ class TestSqlError:
         with pytest.raises(exception):
             sql.raise_sqlite_error("Message", sql_err)
 
+    def test_out_of_memory(self):
+        """On out of memory error, we get an error code '-1' from Qt."""
+        sql_err = QSqlError("driver text",
+                            "out of memory",
+                            QSqlError.UnknownError,
+                            sql.SqliteErrorCode.UNKNOWN)
+        with pytest.raises(sql.SqlEnvironmentError):
+            sql.raise_sqlite_error("Message", sql_err)
+
     def test_logging(self, caplog):
         sql_err = QSqlError("driver text", "db text", QSqlError.UnknownError,
                             '23')