elastic-agent: don't swallow download errors

[axw]

What does this PR do?

Stop swallowing the error from io.Copy when reading from response bodies in the http downloader.

Why is it important?

This prevents storing a partial artifact download, which leads to a permanent error state.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
  - [ ] I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

Move to Australia, run elastic-agent with sourceURI pointed at the staging artifacts URL for 7.11.0 BC1 (https://staging.elastic.co/7.11.0-710164a0/summary-7.11.0.html) :)

(It seems there's no CDN on staging, and it's super slow from here. It's going to be a transient issue, but the last few attempts to fetch apm-server have failed part way through.)

More seriously, you would have to set up something (e.g. a reverse proxy) to inject faults into responses to elastic-agent.

Related issues

None.

Logs

2020-12-22T11:03:32.446+0800    INFO    operation/operation_fetch.go:75 operation 'operation-fetch' downloaded apm-server.7.11.0 into /tmp/zinga/elastic-agent-7.11.0-linux-x86_64/data/elastic-agent-fc48a3/downloads/apm-server-7.11.0-linux-x86_64.tar.gz                      
...
2020-12-22T11:03:32.515+0800    ERROR   log/reporter.go:36      2020-12-22T11:03:32+08:00: type: 'ERROR': sub_type: 'FAILED' message: Application: apm-server--7.11.0[aa633810-4400-11eb-bc64-2b65e7226012]: State changed to FAILED: operation 'operation-verify' marked 'apm-server.7.11.0' corrupted: /go/src/github.com/elastic/beats/x-pack/elastic-agent/pkg/agent/operation/operation_verify.go[77]: unknown error
...
2020-12-22T11:03:37.978+0800    INFO    operation/operation_fetch.go:61 apm-server.7.11.0 already exists in /tmp/zinga/elastic-agent-7.11.0-linux-x86_64/data/elastic-agent-fc48a3/downloads/apm-server-7.11.0-linux-x86_64.tar.gz. Skipping operation operation-fetch
...
2020-12-22T11:03:38.043+0800    ERROR   application/fleet_gateway.go:168        failed to dispatch actions, error: operator: failed to execute step sc-run, error: operation 'operation-verify' marked 'apm-server.7.11.0' corrupted: /go/src/github.com/elastic/beats/x-pack/elastic-agent/pkg/agent/operation/operation_verify.go[77]: unknown error: operation 'operation-verify' marked 'apm-server.7.11.0' corrupted: /go/src/github.com/elastic/beats/x-pack/elastic-agent/pkg/agent/operation/operation_verify.go[77]: unknown error

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[elasticmachine]

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• Build Cause: Pull request #23235 event

• Start Time: 2020-12-22T04:06:40.749+0000

• Duration: 26 min 37 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1422
Skipped	4
Total	1426

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Test stats 🧪

Test	Results
Failed	0
Passed	1422
Skipped	4
Total	1426

[ruflin]

Change LGTM, did not go through the full testing cycle ;-) I assume it is an oversight that we skipped that error.