Incomprehensible exception thrown due to missing var name

[cuebit]

This PR fixes an incomprehensible exception thrown when attempting to register a model that references an absent one (either it was not registered or doesn't exist) or context based retrieval.

Exception thrown:

[Vuex ORM] Could not find the model ``. Please check if you have registered the model to the database.

Previously it stated the suspect model entity but an undeclared variable in the error message prohibits this.

Model definition:

class User extends Model {
  static entity = 'users';
  static fields() {
    return {
      id: this.attr(null),
      role: this.hasOne('nonregistered')
    }
  }
}

Context based:

new Database().model('nonregistered')

$store.$db().model('nonregistered')

[codecov]

Codecov Report

Merging #546 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #546   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          47     47           
  Lines        1835   1837    +2     
  Branches      255    255           
=====================================
+ Hits         1835   1837    +2

Impacted Files	Coverage Δ
src/database/Database.ts	100% <100%> (ø)	⬆️

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[kiaking]

Ahhh good point! Thanks! Awesome.