Required prop for Taxonomy Picker

[AJIXuMuK]

Q	A
Bug fix?	[ ]
New feature?	[x]
New sample?	[ ]
Related issues?	implements #216

What's in this Pull Request?

The PR adds 3 new properties to TaxonomyPicker control:

Property	Type	Required	Description
errorMessage	string	no	Static error message displayed below the picker. Use onGetErrorMessage to dynamically change the error message displayed (if any) based on the current value. errorMessage and onGetErrorMessage are mutually exclusive (errorMessage takes precedence).
onGetErrorMessage	(value: IPickerTerms) => string | Promise<string>	no	The method is used to get the validation error message and determine whether the picker value is valid or not. Mutually exclusive with the static string errorMessage (it will take precedence over this). When it returns string: If valid, it returns empty string. If invalid, it returns the error message string and the picker will show an error message below the picker. When it returns Promise<string>: The resolved value is display as error message. The rejected, the value is thrown away.
required	boolean	no	Specifies if to display an asterisk near the label. Note that error message should be specified in onGetErrorMessage or errorMessage