Propagate errors from Esplora backends

[vladimirfomene]

Quick fix for #47

[notmandatory]

ACK bf2b0c2

Looks good, just one comment that should be cleaned up before merging.

[realeinherjar]

ACK bf2b0c2

Great job. Just remove the println! as @notmandatory suggested.

[coveralls]

Pull Request Test Coverage Report for Build 6363963533

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 80 of 158 (50.63%) changed or added relevant lines in 3 files are covered.
• 7 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-1.3%) to 79.353%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/async.rs	43	74	58.11%
src/blocking.rs	4	51	7.84%

Files with Coverage Reduction	New Missed Lines	%
src/blocking.rs	7	58.6%

Totals
Change from base Build 5409842518:	-1.3%
Covered Lines:	907
Relevant Lines:	1143

---

💛 - Coveralls