Add shebang line and specify error handling options in dist.sh script.

dist.sh

@@ -1,3 +1,4 @@
+#!/bin/sh
 ##===--------- dist.sh - package up binary distribution of IWYU -----------===##
 #
 #                      The LLVM Compiler Infrastructure
@@ -7,6 +8,8 @@
 #
 ##===----------------------------------------------------------------------===##
 
+set -eu
+
 # Create clean dist directory
 rm -rf dist
 mkdir dist