Fix pkg installer not opening BGMApp if relocated.

If the user moved BGMApp and then installed a new version from a .pkg,
BGMApp would be installed to the same place the old version had been
moved to. When pkg/postinstall tried to open BGMApp, it would fail
because it assumed BGMApp would be installed to /Applications.

Also, the installer now fails with an error message if it can't open
BGMApp after finishing the install.

Fixes #164.

package.sh

@@ -28,7 +28,7 @@
 # Based on https://github.com/tekezo/Karabiner-Elements/blob/master/make-package.sh
 #
 
-# TODO: Code signing. See `man pkgbuild`.
+# TODO: Code signing. See `man productbuild`.
 
 PATH="/bin:/sbin:/usr/bin:/usr/sbin:/usr/local/bin"; export PATH
 
@@ -162,6 +162,7 @@ pkgbuild \
 
 productbuild \
     --distribution "pkg/Distribution.xml" \
+    --identifier "$pkg_identifier" \
     --resources "pkgres" \
     --package-path "$out_dir" \
     "$pkg"

pkg/postinstall

@@ -19,7 +19,7 @@
 #
 # postinstall
 #
-# Copyright © 2017 Kyle Neideck
+# Copyright © 2017, 2018 Kyle Neideck
 #
 
 PATH=/bin:/sbin:/usr/bin:/usr/sbin; export PATH
@@ -50,7 +50,14 @@ bash "post_install.sh" "$xpc_helper_path" "BGMXPCHelper.xpc/Contents/MacOS/BGMXP
     killall coreaudiod &>/dev/null) && \
     sleep 5
 
-open "${dest_volume}/Applications/Background Music.app"
+# Try opening BGMApp using its bundle ID first so the installer can declare it as "relocatable".
+# That way, if the user moves BGMApp and then installs a newer version of Background Music, the
+# installer will try to find the old version of BGMApp and put the new one in the same place.
+#
+# If we can't open BGMApp, it very likely didn't install properly, so we fail the install.
+open -b com.bearisdriving.BGM.App || \
+    open "${dest_volume}/Applications/Background Music.app" || \
+    exit 1
 
 # The installer plays a sound when it finishes, so give BGMApp a second to launch.
 sleep 1