Improve validation and error handling

[gilesknap]

Some improvements suggested in conversation with @coretl

• in getattrs make a nice error when a non-existent attribute is requested (I am type X with attrs ... but you have asked for Attr Y)
• also remove need to say object.name instead render object as its id
• Provide a validation capability for defs e.g.

defs:
  - name: Psc
    args:
      - name: carrier
        type: object
        description: IPAC carrier name

      - name: interrupt_vector
        type: object
        description: Interrupt Vector reserved with epics.InterruptVectorVME, count=3

    validate:
      - assert: interrupt_vector.count == 3
        message: Interrupt_vector must be reserved with epics.InterruptVectorVME, count=3

[gilesknap]

Do this after J20

[gilesknap]

The first two bullets are now covered in 3.0.0b5

Leaving this open as the suggestion of a validate is still useful.