Consider dropping error result from ImmutableTree.Iterator

[elias-orijtech]

Right now every caller of ImmutableTree.Iterator must handle its error result, which in turn leads to errors being ignored:

https://github.com/cosmos/iavl/blob/64ffc2749400f762d6279db0fd1ccc2b22e69492/immutable_tree.go#L49

However, the only reason an error can be returned is if IsFastCacheEnabled returns an error, which supposedly is very rare.

A better API is to fall back to NewIterator when IsFastCacheEnabled fails and remove the error result. This benefits every caller, direct and indirect.

CC @odeke-em