Consider including stderr in error from Init

[radeksimko]

One of the nightly E2E tests had failed with the following output:

=== RUN   TestDestroy/basic-0.12.30
    util_test.go:110: [INFO] running Terraform command: /var/folders/6y/gy9gggt14379c_k39vwb50lc0000gn/T/tfinstall342488260/v-0.12.30/terraform init -no-color -force-copy -input=false -lock-timeout=0s -backend=true -get=true -upgrade=false -lock=true -get-plugins=true -verify-plugins=true
    destroy_test.go:16: error running Init in test directory: exit status 2
        2021/04/05 00:22:07 [DEBUG] Using modified User-Agent: Terraform/0.12.30 tfexec-e2etest HashiCorp-terraform-exec/0.13.1

Here exit status 2 is not enough context for further debugging - which is "fine" if it's just a nightly test (although annoying), but it may not be that great for a consumer leveraging Init() and needing to debug similar error.

which makes me wonder if it's worth either:

1. Including stderr in the error at all times (for commands that otherwise don't expose stderr), or
2. Revisiting the idea of custom error types, which was initially binned due to worries about breaking compatibility when exploring ways of resolving Provide better errors caused by context cancellation #107