Fix error handling in open database procedure

When opening a database fails a proper error message is now shown which
hopefully tells the user what the source of the problem is. Note: this
will only work for newer versions of SQLite.

This requires a bug fix which is included in this commit. The error
message wasn't taken from the correct database handle, so it had nothing
to do with the actual error source. This is fixed by this commit as
well.

src/MainWindow.cpp

@@ -307,7 +307,7 @@ bool MainWindow::fileOpen(const QString& fileName, bool dontAddToRecentFiles, bo
                     loadPragmas();
                 retval = true;
             } else {
-                QMessageBox::warning(this, qApp->applicationName(), tr("Invalid file format."));
+                QMessageBox::warning(this, qApp->applicationName(), tr("Could not open database file.\nReason: %1").arg(db.lastErrorMessage));
                 return false;
             }
         }