added failing spec for calling #let w/o a block
Give a useful error when let or subject called without a block.

This is done by nil chcking the block in let before using it.

Signed-off-by: Sam Phippen <samphippen@googlemail.com>
Changelog entry for new subject/let error messages

Signed-off-by: Sam Phippen <samphippen@googlemail.com>
Merge pull request #859 from samphippen/let-without-a-block

Give a better error message when let called without a block.