Improvement: error context reporting

[fredbi]

Errors and warnings are dumped in any order, with poor information about context in the spec.

Since retrieving line numbers is a hard problem, I suggest we could extend the Result object to provide a map of paths linked to errors.

Beware : this would affect the interface with Result (AddError and possibly some more direct usage).

In the end, messages would be pretty printed by sorting and indenting keys.
Not quite line numbers but better than the unordered list we have today.

Also referred by:

• Issue create a spec parser that retains line number information go-swagger/go-swagger#121 - Not solved with line numbers, but improve context reporting using key paths.

[diafour]

messages would be pretty printed by sorting and indenting keys.

It also helps to include an index of item in array.

type: array
items:
  type: object
  properties:
    prop1:
      type: string

[
  { "prop1": "valid"},
  { "prop1": 2989 },
]

Error now:

prop1 in body must be of type string: "number"

Error with index:

[1].prop1 in body must be of type string: "number"