Do not delete user on verification email error

[bquenin]

Fix #1734

How To Test

$ alias amp='amp -k'
$ amp cluster create --registration=email --notifications=true --local-no-logs --local-no-metrics
[localhost:50101]
ampagent image pull failed, which is expected on a development version
2017/10/26 10:41:14 ampctl (version: 0.16.0-dev, build: 37c5dd2f)
2017/10/26 10:41:14 Version test: PASS
2017/10/26 10:41:14 Labels test: PASS
2017/10/26 10:41:14 Using the following path for secrets: /defaults
2017/10/26 10:41:14 Skipping already existing secret: alertmanager_yml
2017/10/26 10:41:14 Skipping already existing secret: amplifier_yml
2017/10/26 10:41:14 Skipping already existing secret: certificate_amp
2017/10/26 10:41:14 Using the following path for configs: /defaults
2017/10/26 10:41:14 Skipping already existing config: prometheus_alerts_rules
2017/10/26 10:41:14 Skipping already existing network: ampnet
2017/10/26 10:41:14 Deploying stack stacks/single/01-core.yml
Creating service amp_etcd
Creating service amp_amplifier
Creating service amp_gateway
2017/10/26 10:41:26 Deploying stack stacks/single/04-proxy.yml
Creating service amp_proxy
{"Swarm Status":"active","Core Services":4,"User Services":0}
$ amp user signup --name bquenin --email bquenin@axway.com --password password
[localhost:50101]
Error: SendGrid API key is empty
$ amp user ls
[localhost:50101]
USERNAME   EMAIL   CREATED ON
bquenin            26 Oct 17 12:41
su                 26 Oct 17 12:41
$ amp user signup --name bquenin --email bquenin@axway.com --password password
[localhost:50101]
Error: user already exists
$ amp user ls
[localhost:50101]
USERNAME   EMAIL   CREATED ON
bquenin            26 Oct 17 12:41
su                 26 Oct 17 12:41
$ docker secret create sendgrid - < ~/amplifier.yml # Where amplifier.yml contains a valid SendGrid key
$ docker service update --detach=false --secret-rm amplifier_yml --secret-add src=sendgrid,target=amplifier.yml amp_amplifier
amp_amplifier
overall progress: 1 out of 1 tasks 
y2rmwmgpew8d: running   [==================================================>] 
verify: Service converged 
$ amp user resend-verification-token bquenin
[localhost:50101]
A new verification email has been sent to bquenin
$ amp -s localhost:50101 user verify eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJUeXBlIjoidmVyaWZpY2F0aW9uIiwiQWNjb3VudE5hbWUiOiJicXVlbmluIiwiQWN0aXZlT3JnYW5pemF0aW9uIjoiIiwiZXhwIjoxNTA5MDE4MTM4LCJpc3MiOiJhbXBsaWZpZXIifQ.I5R3e-axWwLNvejcvh5VVIFuVEOsf8TgdpK2mA73GzA
[localhost:50101]
Your account has now been activated.
$ amp login --name bquenin --password password
[localhost:50101]
Welcome back bquenin!
$

[ndegory]

LGTM