Update pooler provider to throw an exception if the API returns non-OK

[highb]

Prior to this commit the API could fail and return ok: false but
floaty wouldn't recognize that as a failure.
This commit updates the pooler code to check for ok and raise and
error with the body of what the api returns if it's non-truthy.

[highb]

Here's what it looks like:

➜  vmfloaty git:(master) ✗ be bin/floaty modify jovial-athlete.delivery.puppetlabs.net --service vmlocal --lifetime 101
HTTP 400: Failed to modify VMs from the pooler vm/jovial-athlete.delivery.puppetlabs.net. {"ok"=>false, "failures"=>["You provided a lifetime (101) that exceeds the configured maximum of 100.", "You provided a lifetime (101) that will extend the current lifetime past the configured maximum of 100."]}
➜  vmfloaty git:(master) ✗ be bin/floaty modify jovial-athlete.delivery.puppetlabs.net --service vmlocal --lifetime 100
HTTP 400: Failed to modify VMs from the pooler vm/jovial-athlete.delivery.puppetlabs.net. {"ok"=>false, "failures"=>["You provided a lifetime (100) that exceeds the configured maximum of 100.", "You provided a lifetime (100) that will extend the current lifetime past the configured maximum of 100."]}
➜  vmfloaty git:(master) ✗ be bin/floaty modify jovial-athlete.delivery.puppetlabs.net --service vmlocal --lifetime 99
{"jovial-athlete.delivery.puppetlabs.net"=>{"ok"=>true}}
Successfully modified VM jovial-athlete.delivery.puppetlabs.net.
Use `floaty list --active` to see the results.

[highb]

@briancain Hey could you approve/merge this? Apparently I can't self-merge. :D

[briancain]

🎉

[briancain]

Done!

[highb]

Yay thanks!