Made the Error struct Send + Sync

[AxelMontini]

In order to be able to construct anyhow::Errors from tera::Errors, we must make this crate errors Send + Sync.

To do so it's only necessary to change 3 lines of code, but I'm not sure whether this is going to be a breaking change or not.

IMO this is a small but great addition that contributes to the usability of this crate.

Edit:
Fixes #442

[Keats]

Thanks, can you add a test like https://stackoverflow.com/questions/41599921/how-to-define-a-struct-that-is-sendsync-and-is-holding-a-iowrite-member?rq=1 to ensure it's correct?

[AxelMontini]

Now the problem is that the CI fails because a command containing && is being run on windows. Gonna create a PR for that too.

[Keats]

For the Windows failure I'm confused, it passes on https://github.com/Keats/tera/pull/353/checks?check_run_id=267363501

[AxelMontini]

For the Windows failure I'm confused, it passes on https://github.com/Keats/tera/pull/353/checks?check_run_id=267363501

Probably Beta stuff. I'm not even sure where to check for changes in GitHub Actions, but probably something has been changed in the last ~8 days.

PR to fix that too: #459

[Keats]

Thanks!