No trailing slash for build.fan resDirs gives unhelpful msg

[SlimerDude]

A common mistake with build.fan is that users forget to add a trailing slash to a resDir:

resDirs = [`etc`]

Currently this gives a rather unhelpful message in the problems view of:

Internal compiler error, please check Error Log view

Indeed the stack trace in the Error Log view does say:

sys::IOErr: Must use trailing slash for dir: /C:/Projects/f4/etc

but it's not easy to find.