fix: not throw error under watch mode

[susiwen8]

• an error occurs when first build with incremental is true, and can't get the build instance #2280

In watch mode, if the initial content had illegal content, esbuild would throw error and exit. However if user brougt syntax error after, esbuild just notify the error, the process still going, user just change back to correct code, doesn't need to restart process.
So I propose under watch mode, esbuild not throw error.