Add method to check error for specific field

[aryanjaya]

This PR related to issue #10 .

This method allow you to check error for the specific field. Useful when combining it with Vuetify.

Usage example in Vuetify:

<v-text-field
  label="Name"
  v-model="form.name"
  :error="form.hasError('name')"
  :error-messages="form.error('name')"
></v-text-field>

I've tested this method in the following versions

"@inertiajs/inertia": "^0.4.0",
"@inertiajs/inertia-vue": "^0.3.0",
"vuetify": "^2.3.14"

any suggestion is welcome