Show HCL parsing errors and typos

[sethvargo]

This PR makes Vault's UX around bad HCL keys and errors a little bit better by clearly calling out the line number where a violation occurred and the violation itself.

You can take a look at the added tests to understand what the output looks like, but here are some examples:

$ echo 'path "/" { foo = "bar" }' > policy.hcl
$ vault policy-write test policy.hcl
* Failed to parse policy: 1 error(s) occurred:
* path "/": invalid key 'foo' on line 1

$ echo 'path "/" { policy = "nope" }' > policy.hcl
$ vault policy-write test policy.hcl
* Failed to parse policy: path "/": invalid policy 'nope'

$ echo 'path "/" { capabilities = ["read", "foo"] }' > policy.hcl
$ vault policy-write test policy.hcl
* Failed to parse policy: path "/": invalid capability 'foo'

Thanks @jefferai for helping and @mitchellh for teaching me all the HCL insides!