Allow multiple css classes to be specified using customErrorsClass

[maxkrass]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[x] Feature request
[ ] Documentation issue or request
[ ] Support request
[ ] Other... Please describe:

Current behavior

Using the customErrorsClass directive, you can add one css class to the the control-error component. Specifying more than one class like so customErrorClass="classA classB" will result in no class being added to the component.

Expected behavior

The directive should allow for multiple classes to be added. Ideally in said fashion: customErrorClass="classA classB"

What is the motivation / use case for changing the behavior?

Frameworks like TailwindCSS and others usually require multiple css classes on one element to achieve a desired look and feel. So if you're in a utility first project, adding a single css class for multiple conceirns can be a bit of an anti-pattern.

[NetanelBasal]

You're welcome to submit a PR.