Resolve "tfswitch fails in Docker container"

Closes #356 (closed)

Original PR (https://github.com/MatthewJohn/terrareg/pull/4):

tfswitch attempts to install the terraform binary to /app/bin/ but this fails because the directory doesn't exist. Add this directory to the Dockerfile.

In addition, bubble up the error which is passed to the TerraformVersionSwitchError exception, which helped catch this.