Hash calculation may throw exception w/ getModifiedTime(); fix

In Eval there is a calculation of hashes by scanning the elements
of classpath, and getting the last modification time of each
directory. When lastModified() was in use, non-existent elements
would return 0L, but getModifiedTime() will throw an exception
instead (like getLastModifiedTime(), incidentally).
So, we catch the FileNotFoundException and return 0L now as well.

main-actions/src/main/scala/sbt/compiler/Eval.scala

@@ -487,8 +487,12 @@ private[sbt] object Eval {
     if (fs eq null) filesModifiedBytes(Array[File]()) else seqBytes(fs)(fileModifiedBytes)
   def fileModifiedBytes(f: File): Array[Byte] =
     (if (f.isDirectory) filesModifiedBytes(f listFiles classDirFilter)
-     else bytes(getModifiedTime(f))) ++
-      bytes(f.getAbsolutePath)
+     else
+       bytes(try {
+         getModifiedTime(f)
+       } catch {
+         case _: java.io.FileNotFoundException => 0L
+       })) ++ bytes(f.getAbsolutePath)
   def fileExistsBytes(f: File): Array[Byte] =
     bytes(f.exists) ++
       bytes(f.getAbsolutePath)