Panic when attempting to configure invalid cache

[SuperQ]

Instead of defaulting to cache type memory when misconfigured, panic.

Signed-off-by: Ben Kochie [email protected]

Looks like this:

$ trickster -conf invalid.conf
time=2018-10-25T12:19:25.109494354Z app=trickster caller=trickster/main.go:63 level=info event="application startup" version=0.1.1
time=2018-10-25T12:19:25.109628973Z app=trickster caller=trickster/metrics.go:47 level=info event="metrics http endpoint starting" address= port=8082
panic: Invalid cache type: "boom"

goroutine 1 [running]:
main.getCache(0xc000032580, 0xc00000e0d0, 0xc00000e0d8)
	/home/ben/go/src/github.com/comcast/trickster/cache.go:49 +0x3c5
main.main()
	/home/ben/go/src/github.com/comcast/trickster/main.go:68 +0x32a

[juliusv]

Ok as a short-term fix, but it'd be much better to exit cleanly with an error rather than outright panic-ing.

[SuperQ]

Yes, a clean exit would be better.