Fix missing interpolation in error message

[Tony Sloane]

In Interpreters.scala this code (line 175):

Failure( new Exception( s"l.flatMap(_._2.messages)" ) )

is missing the ${ } around the content of the message.

I’m not sure of the precise fix, but this seems to work

Failure( new Exception( l.map( _._2.toString ).mkString( ", " ) ) )

to produce multiple lines, one per failure. But perhaps something else is better…