Print an informative message when trying to register an unsupported class

Previously, if there was a class name passed in the core_native_classes
or graphics_native_classes system property, and the class name wasn't
present in the JNI registration map, the process would crash, and you
would see the generic error message:

    out_of_range was thrown in -fno-exceptions mode with message
    "unordered_map::at: key not found"

Update the logic to print an actionable error message and return an
error, which causes JNI registration to fail.

Bug: 437898369
FLAG: EXEMPT host-only
Test: Robolectric test in Google3
Change-Id: I370a01ae3fe9aee4b0d023bbd4406a7d07afc456