fix #1222 improve SelectBox's wrong value error message & doc

When the `options` array didn't contain the value the developer was trying to set
via `setValue`, the message in console was a bit cryptic. This commit adds
more explanations in the console error and cfgbeans.
Actually, I would be more generic here: I would just say that the value of the SelectBox must be one of the entries of this array. The usecase in which the developer sets the value before the options is quite marginal...