Confused error message when ti.init() is not called

[erizmr]

Describe the bug Confused error message when ti.init() is not called. It does not remind the user that ti.init() should be called first.

To Reproduce

import taichi as ti

a = ti.field(dtype=float, shape=())

Log/Screenshots

Related issue: #784

[StormMelody]

Is your problem solved? I meet the same problem, too.