Support class for ExPropertyEditor which provides means for validating
hints from the PropertyEnv instance passed to attachEnv. Forces
subclasses to be fail-fast in the case that illegal values are passed
via the PropertyEnv (the alternative is cryptic error messages when
the editor tries to use the hints).
Implementation of PropertyEditorSupport.attachEnv(). This method
is final to ensure that the values from the env are validated.
Subclasses should override attachEnvImpl to provide the actual
attaching behavior. attachEnvImpl is called first, then
validateEnv (to avoid fetching the values twice).
Validate values stored in the PropertyEnv. This method allows
subclasses to be fail-fast if they are supplied illegal values
as hints from the PropertyEnv. Subclasses should confirm that any
hints used by their property editor are valid values. If they
are not valid, an EnvException should be thrown with a clear
description of the problem.