Provide better errors caused by context cancellation

https://github.com/hashicorp/terraform-exec/pull/91 was closed partially because we were worried about implications in terms of backwards incompatibility.

I do however believe that the library should provide decent error messages - i.e. _not_ `context deadline exceeded` _by default_, so I'm hoping this can be revisited before v1 as that gives us chance to make some breaking changes.

Start by reviewing the closed pull request #91 and its discussion, then trace the library's context-cancellation error handling. The change is done when cancellation produces useful errors rather than context deadline exceeded by default, with the backwards-compatibility implications addressed before v1.

Written by the indexing model from the issue text.