crypto/x509: add more detail to the error message arising from a missing hash function.

[agl]

If a certificate is signed with a hash function that wasn't compiled in the error
message isn't bad, but could be better:

x509: certificate signed by unknown authority (possibly because of "x509: cannot
verify signature: algorithm unimplemented" while trying to verify candidate
authority certificate "CA Cert Signing Authority")

[rsc]

Comment 1:

Labels changed: added release-go1.5, removed release-go1.4.