Add Input.error property

[itay-raveh]

Added an error property to the Input element which always contains the latest error message from the given validators.

Motivation

Say we have the following ui.input setup:

inp = ui.input(label=..., validation={'Input too long': lambda value: len(value) < 20})
def on_submit():
    ... # content of following code blocks here
inp.on("keydown.enter", on_submit)

Say in the on_submit callback we want to base behavior on the error state.

How would that look before and after this PR?

Checking if an error is active

Before

# access to "private" member :(
if "error-message" in inp._props:
    ... # do something with `inp.value`

After

if inp.error:
    ... # do something with `inp.value`

Checking if a specific error is active

Before

# access to "private" member :(
if inp._props.get("error-message") == "Input too long":
    ... # do something with `inp.value`

After

if inp.error == "Input too long":
    ... # do something with `inp.value`

[falkoschindler]

I finally looked into your PR, @itay-raveh!

In order to use this feature across all input elements with validation (input, number, textarea), I refactored it into a mixin. This nicely separates the validation code from other concerns.

Regarding the change to validation being an Optional[Dict] rather than a possibly empty Dict: I'm not quite convinced that we need this change. Therefore I switched back to a Dict. But maybe there is an argument I missed.

And I'm thinking about whether the property name error is to generic and should be something like validation_error. Once we release a new feature with a certain name, it's always tricky to change it later.

[falkoschindler]

Oh, there's another point we need to think about: How to integrate error with ui.number's min/max limits? There has been a feature request #924 for an is_valid property, which might combine validation, min and max. But how does it integrate with an error of type Optional[str]?

Maybe we simply add out_of_limits to ui.number and the user can decide which one to use, e.g.

i = ui.number(min=0, max=100, value=50, validation={'Too small!': lambda value: value > 80})
...
if i.error or i.out_of_limits:
    ui.notify('Something is wrong.')