Fix for allowing compile error message of less compiler to be a ConsString also.

I had the case in which the less task was issuing the following error when compiling and a variable wasn't defined properly in less:
[error] (frontend/compile:less) java.lang.ClassCastException: org.mozilla.javascript.ConsString cannot be cast to java.lang.String

After this fix the error message is displayed correctly:
[error] (frontend/compile:less) sbt-less error: components.less [44,20]: variable @lightGreyBG is undefined

Some errors still returned String as error message so the match seems necessary to accomodate both.

I would be glad if this would make it into the 0.8 release as I was getting this error quite frequently.

Kind regards,
Roland