Fail gracefully when apek_key is not found

When the private key file can not be obtained, e.g., due to missing git
project where the apex_key module is defined, fail gracefully with an
error message instead of panicing.

Bug: 119256776
Test: modify the key property in /systme/apex/Android.bp to
apex.test.key2 and run `ALLOW_MISSING_DEPENDENCIES=true m apex.test`
The build fails with

error: system/apex/Android.bp:32:8: module "apex.test" variant "android_common_apex.test": key: private_key for "apex.test.key2" could not be found

Change-Id: I2cd5fed0f046de1f0b380fbdac5f8b82816b0e24