Fix 500 when use a duplicate email instead of giving an error tip

[lunny]

This will part to resolve #13 . And This also formatted the code by my vscode go plugin.

[strk]

Please don't mix functional and style changes.
The code you add seem to be suffering from race conditions, that is the mail might be non-existent at time of checking but be inserted later, at time of user update/insert transaction

[lunny]

I will split this into 2 PRs.

[andreynering]

Trusted LGTM

[appleboy]

LGTM