Prevent panic on failed config load

[mattbostock]

Prevent Alertmanager from panicking when the configuration cannot be
loaded.

Spotted in version 0.4.2:

INFO[0000] Starting alertmanager (version=0.4.2, branch=HEAD, revision=9a5ab2fa63dd7951f4f202b0846d4f4d8e9615b0)  source=main.go:84
INFO[0000] Build context (go=go1.7.3, user=root@45f28166fed1, date=20170117-13:50:50)  source=main.go:85
INFO[0000] Loading configuration file                    file=alertmanager.yml source=main.go:156
ERRO[0000] error: yaml: line 64: could not find expected ':'  source=api.go:115
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x401adb]

goroutine 1 [running]:
panic(0x83e240, 0xc42000e020)
        /usr/local/go/src/runtime/panic.go:500 +0x1a1
main.(*API).Update(0xc4200b20f0, 0xc42019f800, 0x17e7, 0x45d964b800)
        /go/src/github.com/prometheus/alertmanager/api.go:117 +0xcb
main.main.func3(0x0, 0x0)
        /go/src/github.com/prometheus/alertmanager/main.go:172 +0x226
main.main()
        /go/src/github.com/prometheus/alertmanager/main.go:192 +0x97a
make: *** [run] Error 2

[fabxc]

Thanks 👍