snmp: return error on unknown conversion type

[phemmer]

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Since the influxdb models marshaller panics on byte slices (when not a valid line-protocol value), we should throw an error instead of passing through the value unchanged.

I didn't update the changelog or anything as this isn't really a bug fix or enhancement. Unless people have misconfigured telegraf, there should be no behavior change.

closes #1852

[phemmer]

I'm also wondering if we should capture panics coming from m.pt.Fields(). Just as protection from any other plugins which might send data that can't be marshalled.

I find it rather odd that a panic is thrown instead of returning an error. I know the current signature of the call doesn't return error, but maybe it should.

[sparrc]

Doesn't seem like it should be panicking, but we are using a fairly old changeset of the influxdb repo, and the latest might have a different behavior.

I'll be updating the dependency in #1777

[phemmer]

Doesn't look like the current code is any different: https://github.com/influxdata/influxdb/blob/master/models/points.go#L1769