fix(send): improve error logging (#208) r=jrgm
fix(send): improve error logging (#208) r=jrgm

* fix(send): improve error logging

Fixes #207

* fix(log): switch from trace to info for email success

* fix(style): adjust sendMail promise

eoger/fxa-auth-server,mozilla/fxa-auth-server,vladikoff/fxa-auth-mailer,mozilla/fxa-auth-server,vladikoff/fxa-auth-mailer,eoger/fxa-auth-server,eoger/fxa-auth-server,mozilla/fxa-auth-server,eoger/fxa-auth-server,mozilla/fxa-auth-server,vladikoff/fxa-auth-mailer