Better error messaging for invalid relative paths on write

[ryanmcnamara]

I tried to write a generic secret to an invalid path (it was interpreted as a relative path) was not great. I ran into this with the client go api, but here it is illustrated just the same with the cli (distilled down)

vault write secret/foo..bar "a=b"
Error writing data to secret/foo...bar: Error making API request.

URL: PUT http://127.0.0.1:8200/v1/secret/foo..bar
Code: 400. Errors:

* internal error

The logs contain

2017/08/14 18:24:36.162348 [ERROR] core: failed to run existence check: error=existence check failed: key cannot be relative path

A command like

vault write secret/foo.bar "a=b"

works just fine immediately following

Some issues:

1. The logs say that the "key" cannot be a relative path, but it is in fact the path that is a relative path here, not one of the keys from the map. Terminology taken from here: https://www.vaultproject.io/api/secret/generic/index.html#create-update-secret
2. The response is a 400 and yet "internal error" is in the content of the response
3. One could potentially argue that "foo..bar" is not a relative path because the ".." does not come after a "/" For example the file system behaves differently (perhaps better):

➜  tmp mkdir x
➜  tmp cd x
➜  x mkdir a..b
➜  x ls
a..b
➜  x