Make `-skip-errors` default behaviour when executing batch specs

Created by: mrnugget

Turning a Slack message into a ticket. Here's what I wrote:

Random thought after writing a lot of batch specs in the past few weeks: I think -skip-errors should be the default behaviour for src batch [preview|apply] and instead of only printing those errors on the CLI we should upload them so that users can see them in the web UI.

Expanding on that, the thinking behind this is that it's a huge pain when your multi-hundred changeset batch spec execution fails due to a single error and you have to retry. Yes, there's a cache, but still.

Probably no one wants the execution to completely fail on error. Users most likely want to be notified about an error and see that in the UI so they can decide whether to abort manually or not.

(I added the src-cli and the batchers/server-side labels because this is about src-cli but will most likely be the default behaviour for server-side batch changes where we won't fail all jobs if a single one failed. Or at least we should make that the default behaviour.)