File upload validation error instead of warning

[srikanthgurram-17]

@stefanseifert We would like to show error validation message instead of warning message whenever the asset media is invalid in file upload dialog selection. is it possible to have a hook inside MediaFormatValidate showAlert method for the consumer to customise to a error instead of warning?

[stefanseifert]

yes, this would be a good addition. i think we can build in this feature into the media handler Granite UI components, with the option to enable it via a configuration parameter.

[stefanseifert]

PR: #69