chore(bin/leanpkg): return nicer error message if greadlink is not available on OSX

see issue #1788

bin/leanpkg

@@ -1,7 +1,15 @@
 #!/usr/bin/env bash
-if command -v greadlink >/dev/null 2>&1; then
-    # macOS readlink doesn't support -f option
-    READLINK=greadlink
+
+unamestr=`uname`
+if [[ "$unamestr" == 'Darwin' ]]; then
+    # OSX
+    if command -v greadlink >/dev/null 2>&1; then
+        # macOS readlink doesn't support -f option
+        READLINK=greadlink
+    else
+        echo "OSX 'readlink' command does not support option '-f', please install 'greadlink'. If you use 'brew', you can install 'greadlink' using 'brew install coreutils'"
+        exit 1
+    fi
 else
     READLINK=readlink
 fi