MimeMapImpl: Tweak message exception when resource not found.

Class.getResourceAsStream(String) returns null rather than throwing
IOException if the resource is not found. This CL tweaks the code
such that the RuntimeException that ends up being thrown always
specified which resource wasn't found, even when the cause itself
was a RuntimeException (such as NPE).

Test: Manually changed the code to specify an incorrect resource
      path, checked that a helpful exception was logged during
      boot.
Bug: 136256059

Change-Id: I4d89f602036bce806eafa925ec09f29a1e683162
1 file changed