Investigate way to delay showing string validation errors

[liammulh]

In phetsims/qa#878, @DianaTavares said:

It is super cool the error message of curly brace, but I feel it aggressive that I just start to write the translation and the error in red is there the hole time. I suggest that the error appears when user move to another text and the translation is incorrect.

[liammulh]

On the one hand, I see how this can be aggressive, but on the other hand I feel it's important for translators to have some indication that they are translating a field that needs special attention. (Similar to @DianaTavares's request in #354.)

[liammulh]

I don't think we are going to do this. We can re-open if we get emails from translators who find it irritating.