dumbwallet: better error messages for generating key

Running 'dumbwallet setup' on Fedora 21 gave me error
"Coud not generate key". The reason is that Fedora's
OpenSSL does not currently support secp256k1.

This patch fixes the above typo and explains the user
why the key generation failed.

Signed-off-by: Tero Roponen <tero.roponen@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>