Report proposal errors earlier and more precisely where possible

[daira]

This follow-up to #1257 is not essential, but it improves the error reporting by:

• reporting some errors when a proposal is constructed rather than when it is used to create transactions;
• reporting more precise errors for unsupported operations.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 32.60870% with 31 lines in your changes missing coverage. Please review.

Project coverage is 60.67%. Comparing base (f35a894) to head (46f1e7b).

Files	Patch %	Lines
zcash_client_backend/src/proposal.rs	31.25%	22 Missing :warning:
zcash_client_backend/src/data_api/error.rs	25.00%	6 Missing :warning:
zcash_client_backend/src/data_api/wallet.rs	0.00%	2 Missing :warning:
zcash_client_backend/src/fees.rs	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1441 +/- ## ========================================== - Coverage 60.73% 60.67% -0.06% ========================================== Files 140 140 Lines 16467 16475 +8 ========================================== - Hits 10001 9996 -5 - Misses 6466 6479 +13 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[daira]

This is not up-to-date with #1257. I will rebase it when that merges.

This is now rebased onto main.