fix panics when API calls fail

[BeryJu]

every so often my instance will panic due to some network hiccup:

2024/02/22 21:22:42.704302 ERROR RESTY Get "https://api.linode.com/v4/account": read tcp 10.244.0.202:42616->88.221.221.99:443: read: connection reset by peer, Attempt 1
{"level":"info","msg":"[002] Get \"https://api.linode.com/v4/account\": read tcp 10.244.0.202:42616-\u003e88.221.221.99:443: read: connection reset by peer","time":"2024-02-22T21:22:42Z"}
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x60 pc=0x7e5b3e]

goroutine 142 [running]:
github.com/DazWilkin/linode-exporter/collector.(*AccountCollector).Collect(0xc000234300, 0xc0001e7f60?)
	/linode-exporter/collector/account.go:57 +0xde
github.com/prometheus/client_golang/prometheus.(*Registry).Gather.func1()
	/go/pkg/mod/github.com/prometheus/client_golang@v1.2.1/prometheus/registry.go:443 +0x10d
created by github.com/prometheus/client_golang/prometheus.(*Registry).Gather
	/go/pkg/mod/github.com/prometheus/client_golang@v1.2.1/prometheus/registry.go:535 +0xb0c

This PR should prevent this by returning early when an API Client returns an error

[DazWilkin]

Ugh, thanks for catching all these.

That's poor form on my part.