fix: don't throw error when currentEra is greater than 0, less than maxErasToCheck

[alecdwm]

The code here would check e.g. eras [-10, -9, -8, -7, -6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5] if currentEra is 6.

Attempting to make calls to the chain with an era parameter less than 0 will throw errors.

Now, we'll filter for negative values, making the list of eras to check just [0, 1, 2, 3, 4, 5] if currentEra is 6.