printflags: improve protobuf decode error message

Include the file path and first bytes of file in the error message in
case aconfig fails to parse one of the protobuf files.

Example output:

  $ adb shell printflags
  Error: failed to parse /vendor/etc/aconfig_flags.pb ([0a], 1 byte(s))

  Caused by:
      Unexpected EOF

Bug: 304278614
Test: atest printflags.test
Change-Id: I18ff88bd25d72dd477c4b11a32505e75884906ee