One validation error instead of all

[foxcool]

At some point, goswagger stopped returning all validation errors when processing the request, but only the first.

[fredbi]

@foxcool go-swagger generated server fails fast and does not attempt to report all validation errors on incoming requests.

If it did in the past, that must have been pretty long ago...
At some point we have discussed having this as a kind of "debug opt-in" in codegen, but eventually decided to keep it simple with only the fail-fast behavior.