Don't include file name in error messages when it is a single dash
Don't include file name in error messages when it is a single dash

Issue #827

ternjs/acorn,marijnh/acorn,TimothyGu/acorn,marijnh/acorn,ternjs/acorn,marijnh/acorn,TimothyGu/acorn,TimothyGu/acorn,mysticatea/acorn,ternjs/acorn,mysticatea/acorn,mysticatea/acorn