Return an error from NodeEntropy::from_bip39_mnemonic in bindings

[kaloudis]

The uniffi-exposed from_bip39_mnemonic constructor was infallible and took the Mnemonic custom type, which parses the string during argument lifting. An invalid mnemonic would fail the lift and surface as an unexpected-error call status. The generated Swift wrapper for an infallible function wraps the call in try!, so passing an invalid mnemonic (eg. a user typo during wallet restore) aborts the process with an uncatchable EXC_BREAKPOINT. Kotlin and Python raise their generic internal exceptions instead.

Following the from_seed_bytes precedent, give the constructor a uniffi-specific signature that takes the mnemonic as a plain string and returns Result<NodeEntropy, EntropyError>, parsing inside the function and reporting failures via a new
EntropyError::InvalidMnemonic variant. The non-uniffi Rust API is unchanged.

Discovered in an ZEUS iOS user crash report.

[ldk-reviews-bot]

I've assigned @tnull as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[tnull]

Ugh, sorry. We'll be working towards getting rid of all custom type to avoid these cases, seems we missed this so far :/

I think we should however do #1059, i.e., expose Mnemonic as a proper interface to bindings users, which also exposes more functionality and allows us to clean up some legacy code.