improve error handling in targeted_long for too-big and too-small targets

[dpaiton]

There are bounds on the target_rate argument that are not observed presently. The rate upper bound is obvious -- they should not be targeting a rate that is greater than the current fixed rate, since longs can only reduce the rate. The lower bound is trickier -- in addition to the base amount being bound by the output of calculate_maximum_long, we can assume that there is a point near zero where the fixed rate is too small.

As of now if you provide pathological arguments for target_rate it will throw errors that are not sufficiently informative; it would be better to check that the target_rate argument is within a reasonable bounds.

[dpaiton]

delvtech/hyperdrive#956 partially addresses this by adding a check if the target_rate is above the current_rate (which means we would want a targeted short).