Specify which attribute is missing on which Entity

[jhollinger]

In the latest release, the ArgumentError resulting when an Entity exposes a non-existing attribute (due to typos, merges, pending database migrations, etc.) gives no context as to which attribute is missing, or even which Entity is to blame (including in the backtrace). In master I notice it now gives a NoMethodError. That's arguably an improvement, but it also tends to even further obscure that grape-entity has anything to do with the error.

In short, finding the Entity and attribute responsible for these errors is often difficult and tedious, especially in large codebases. I think a simple change to the exception message will make them much easier to find and fix. Something along the lines of NoMethodError: Entities::V1::Session missing attribute 'foo' on #<User:0x007f787ed1c7b8>.

Message in 0.4.4

ArgumentError: ArgumentError
    /home/jhollinger/.gem/ruby/2.1.0/gems/grape-entity-0.4.4/lib/grape_entity/entity.rb:557:in `delegate_attribute'
    ...

Message in master

NoMethodError: undefined method `foo' for #<User:0x007f9b0d22b2a8>
    /home/jhollinger/.gem/ruby/2.1.0/gems/activemodel-4.1.8/lib/active_model/attribute_methods.rb:435:in `method_missing'
    ....

Example proposed message

NoMethodError: Entities::V1::Session missing attribute `foo' on #<User:0x007f787ed1c7b8>
    /home/jhollinger/.gem/ruby/2.1.0/gems/grape-entity-0.4.4/lib/grape_entity/entity.rb:560:in `rescue in delegate_attribute'
    ...

[dblock]

Perfect, merging, thx.

[swistaczek]

👍 great one @jhollinger