canRun should return false on scala linter instead of raising an exception

Summary:
This was causing the following error in environments that didnt have scala configured:

Some linters failed:

• ArcanistScalaSBTLinter: ArcanistUsageException: This directory does not appear to be maintained by SBT, as we can't seem to find a working build file (project/Build.scala or build.sbt).

Test Plan: .

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4070