Enrich errors caused by context cancellation

[radeksimko]

Previously the following context.* errors would be returned when the Terraform execution can't start because the context passed was already cancelled.

context deadline exceeded

context canceled

This is being replaced with more informative custom ExitError, e.g.

["/path/to/0.13.1/terraform" "version"] exited: context deadline exceeded

---

The following exec.ExitError error would be returned when execution is interrupted due to context (cancellation or deadline):

signal: killed

which is being replaced with ExitError too

["sleep" "1"] (pid 71492) exited (code -1): signal: killed
context deadline exceeded

---

Importantly the ExitError implements Unwrap to remain comparable with the original context errors or exec.ExitError, as demonstrated in attached tests.

---

We can make deadline errors even more informative by making timeout a first-class concept and implementing custom context.Context which reflects that. Once that is done, the error handling here should in theory just pick it up - only tests may need to be updated. I'll raise a separate PR for this though.

[radeksimko]

Closing in favour of #94

[radeksimko]

#94 does simplify error handling for consumers, but it doesn't actually address any of the above examples of errors.

I think there's still value in this PR, but also I understand if maintaining backwards compatibility for those who don't use errors.Is/As is important, especially if we still maintain Go 1.12 / 1.13 compatibility.

I still think that that default error messaging should be improved and shouldn't be left to each consumer, but maybe it's a topic to revisit later, before we cut v1 and have better justification to make such (potentially) breaking change.