Allow option to abort validation after encountering first error

[judah-w]

Proof of concept of how to resolve this vulnerability: Performance: Linear response time degradation when processing large numbers of validation errors

I have:

• Added tests covering the bug / feature
• Updated any relevant documentation

(Will complete these later if the overall approach is sound)

[coveralls]

coverage: 87.227% (-0.3%) from 87.484%
when pulling 8b96fa0 on judah-w:stop-walk-on-error
into 4956970 on vektah:master.

[vijaykramesh]

+1 thanks for the fix!

[StevenACoffman]

I really appreciate you making a PR for this!

As I mentioned on the issue, rather than a boolean, I think it will be more generally useful as an integer for the maximum number of errors, where 0 is unlimited, 1 would do as this currently does, but you could specify some reasonable number like 10, which would cover 90% of normal validation problems with more useful feedback, but still prevent the DoS behavior.