Revisit inline validation message styling

[mikejolley]

This came up when fixing #4582

Right now the validation messages are positioned absolutely.

This prevents a jump in content when validation errors are shown, but comes at the cost of only supporting errors up to 2 lines. Any more than this and it overlaps other content.

Since we cannot be sure there will only ever be 2 lines of error (this can be impacted by font size, display width, mobile, error text) it seems like a fragile approach we should revisit at some point.