pi: Make status transition error human readable.

[amass01]

This adds human readable billing statuses to the pi plugin and uses them
in the invalid billing status transition error.

---

Closes #1541.

[amass01]

$ pictl proposalsetbillingstatus 309812a1c6617320 active
400 pi plugin error code 15, billing status change is not allowed: invalid billing status transition, active to active is not allowed

[lukebp]

ACK

Add unit tests.

[lukebp]

This needs a unit test veryifying that all statuses have a human readable entry. Format it like this please.

	// ErrorCodeLast is used by unit tests to verify that all error codes have
	// a human readable entry in the ErrorCodes map. This error will never be
	// returned.
	ErrorCodeLast ErrorCodeT = 23

	err := unittest.TestGenericConstMap(ErrorCodes, uint64(ErrorCodeLast))
	if err != nil {
		t.Error(err)
	}

[lukebp]

ACK