age-keygen -y should report error if it fails to parse input

[vbezhenar]

Right now it does not do that which is confusing:

% echo test | age-keygen -y
age-keygen: error: failed to parse input: error at line 1: malformed secret key: separator '1' at invalid position: pos=-1, len=4
age-keygen: error: no identities found in the input
% echo $?
0

[FiloSottile]

Yep, that's a bug, thank you!