Consider dropping unwrap_or(1.0) from convert_fee_rate

[tnull]

Currently, when something goes wrong/the corresponding pair can't be found, convert_fee_rate will default to a bogus 1.0 fee rate:

rust-esplora-client/src/lib.rs

Lines 90 to 94 in 7faab65

	pairs
	.into_iter()
	.find(|(k, _)| k <= &target)
	.map(|(_, v)| v)
	.unwrap_or(1.0)

This however messes with users' assumptions they depend on receiving a correct fee update (e.g. in Lightning). Rather than returning a (btw. undocumented) default value, convert_fee_rate should just error out if something goes wrong, as it's fallible anyways.

[tnull]

Mhh, one caveat seems to be that Esplora on Signet seems to return {} for fee-estimates. However, arguably falling back to 1 sat/vbyte should still be part of the user-side logic, i.e., a conscious decision?

[notmandatory]

I agree this shouldn't default to 1 sat/vB, it should throw an error so the caller can decide if they want to retry or default to some value.

[tnull]

Yes, I also agree that this would be the preferable behavior, although want to note that changing it to return an error will let user's fee estimation on Signet fail where it used to work nicely before. So if the change is introduced it would be nice to feature it prominently in the release notes to make users aware of the fix. It might also be worth breaking the API for this to force users to double-check their code.

[ValuedMammal]

Considering that producing a value is contingent on find, perhaps returning Option<f32> would be appropriate.

[notmandatory]

Returning None instead of an Err makes sense to me. The logic being that you asked for the fee rate for a given target and one didn't exist in the values returned by the esplora server. Then the caller can decide to use some default or not.