Improve error message for Unknown error trying to build pom

When adding a project, if the pom is invalid, the user sees only:

add.project.validation.error
Unknown error trying to build POM

The first line looks like a value is missing for 'add.project.validation.error' in the i18n bundle.

In this case, the project had a couple of jar dependencies marked with <scope>import</scope> which is not allowed. (Import scope can only be used with dependencies of type pom.)

The error was available in the server log. It should be displayed to the user.