adb: log more detail on failure to read keys.

Before we just got "Failed to read key". After:

  adb E 10-07 08:20:14 258249 258249 auth.cpp:176] Failed to read key \
    from '/usr/local/google/home/enh/.android/adbkey'
  94390117965240:error:0900006e:PEM routines:OPENSSL_internal:NO_START_LINE:\
    external/boringssl/src/crypto/pem/pem_lib.c:622:Expecting: ANY PRIVATE KEY

Also fix the misleading "Failed to generate" message from adb_auth_init.

Bug: http://b/141715453
Test: manually corrupted key; see above
Change-Id: I6732ee6b683c8548d596d7c22eeddab8ce9a3cea