litewitness: Don't exit if signing client certificate fails

[niels-moller]

Fixes #55

[niels-moller]

When running tests locally, I get this failure that looks unrelated:

--- FAIL: TestScript (0.00s)
    --- FAIL: TestScript/bastionlocalhost (6.29s)
        testscript.go:584: # gentest seed b4e385f4358f7373cfa9184b176f3cccf808e795baf04092ddfde9461014f0c4
            # set up log (0.008s)
            # start bastion with host:port -listen-http (5.048s)
            > exec litebastion -testcert -backends=backends.txt -listen localhost:8445 -listen-http localhost:8081 &litebastion&
            > waitfor localhost:8445
            FAIL: testdata/bastionlocalhost.txt:8: timeout waiting for localhost:8445: dial tcp [::1]:8445: connect: connection refused
            [background] litebastion -testcert -backends=backends.txt -listen localhost:8445 -listen-http localhost:8081: exit status 2
            [stderr]
            flag provided but not defined: -testcert
            Usage of /tmp/testscript-main1526288427/bin/litebastion:
              -backends string
                        file of accepted key hashes, one per line, reloaded on SIGHUP
              -cache string
                        directory to cache ACME certificates at
              -email string
                
              -home-redirect string
                        redirect / to this URL
              -host string
                        host to obtain ACME certificate for
              -listen string
                        host and port to listen at (default "localhost:8443")
              -listen-http string
                        host:port or localhost port to listen for HTTP requests
              -tls-cert string
                        path to TLS certificate; disables ACME
              -tls-key string
                        path to TLS private key; disables ACME
            
FAIL
FAIL    filippo.io/torchwood/cmd/litewitness    6.303s

[FiloSottile]

When running tests locally, I get this failure that looks unrelated:

Sorry, my bad, I had broken main. Fixed!