catch sandbox errors from backend api
clarify bootstrapping failure message

- clearly point out bootstrap.log that can be examined for
  troubleshooting why the setup failed
be less heavy handed in secure_mysql_installation bootstrapping

- only remove anonymous users
- reset all root@<any host> passwords
always add root@localhost to new sandbox instance
fix bug with logging start time

- start variable didn't exist after the logic creation was moved
  into a separate function
log message about injecting root@localhost user into sandbox

- this extracts out the dml from mysql_system_tables_data.sql
  and changes this to a REPLACE INTO so it runs regardless
  of whether we created new tables.  So even if a user provides
  their own initial data from a tarball backup and a root
  user does not exist, a new root@localhost user will be added.
don't purge all mysql.db users

- now secure_mysql_installation.sql only purges the test and test_%
  db privileges.