(PUP-1898) Fix broken exception raising when inline_epp gets non string

This was a copy/paste error from the epp() function and it
referenced the non existing "file" variable instead of the source
string variable.
(PUP-1898) Fix reference to ArgumentException

This should have been ArgumentError
(maint) Remove dead code checking non existing ImportExpression

ImportExpression was removed some time ago, but the validation
of it remained in the 4.0 checker.
(maint) Remove dead-code check if ImportExpression is r-value

ImportExpression does not exist since some time back. This is dead code.
(PUP-1897) Allow EPP render expression to render a block

If a block is given in <%= {} %> the block should be evaluated, and
what it produces should be rendered. This commit makes this possible
by:
* explicitly handle the render block case
* allowing {} to be validated as r-value producing
Merge pull request #2423 from hlindberg/pup-1897_epp-render-block

(PUP-1897) Allow EPP render expression to render a block
Merge branch 'master' of https://github.com/puppetlabs/puppet