Improve message when directory is passed dotc.

Previously, running ./bin/dotc <directory> caused a null pointer
exception. Now, the error is:

  error: expected file, received directory 'bin'

1 files changed, 7 insertions, 3 deletions

diff --git a/src/dotty/tools/dotc/Run.scala b/src/dotty/tools/dotc/Run.scala
index fa69530b5..f5ba56a7e 100644
--- a/src/dotty/tools/dotc/Run.scala
+++ b/src/dotty/tools/dotc/Run.scala
@@ -29,10 +29,14 @@ class Run(comp: Compiler)(implicit ctx: Context) {
   var units: List[CompilationUnit] = _
 
   def getSource(fileName: String): SourceFile = {
-    val encoding = ctx.settings.encoding.value
     val f = new PlainFile(fileName)
-    if (f.exists) new SourceFile(f, Codec(encoding))
-    else {
+    if (f.isDirectory) {
+      ctx.error(s"expected file, received directory '$fileName'")
+      NoSource
+    } else if (f.exists) {
+      val encoding = ctx.settings.encoding.value
+      new SourceFile(f, Codec(encoding))
+    } else {
       ctx.error(s"not found: $fileName")
       NoSource
     }