Use .attr() when evaluating errors in validations

[airhadoken]

#410 in pull request form

In order to make it possible to bind a can.compute to error evaluations, switch the [attr] access of attributes to .attr(attr)

I've been using this for a few weeks on reciprocity/ggrc-core and it seems to be working out without unintended side effects.

[daffl]

I think that makes total sense. Lets get it into 1.1.7. Does this need a test?

[bmomberger-reciprocity]

👍 thanks for constructing the test.