Make artifact retrieval error messages actionable.

When artifact retrieval fails, it gives inscrutable errors like

    2021/05/24 13:45:34 Initializing java harness: /opt/apache/beam/boot \--id=1-2 \--provision_endpoint=localhost:33957
    2021/05/24 13:45:44 Failed to retrieve staged files: failed to retrieve /tmp/staged in 3 attempts: failed to retrieve chunk for /tmp/staged/beam-sdks-java-io-expansion-service-2.28.0-oErG3M6t3yoDeSvfFRW5UCvvM33J-4yNp7xiK9glKtI.jar
    caused by:
    rpc error: code = Unknown desc = ;
 
Also, we should probably make it more clear that there is a causal relationship between "Docker container x logs" and "No container running for id x" https://github.com/apache/beam/blob/873201205a6502e7f44f9e398dc7558710b26b12/runners/java-fn-execution/src/main/java/org/apache/beam/runners/fnexecution/environment/DockerEnvironmentFactory.java#L137-L148
 

Imported from Jira [BEAM-12392](https://issues.apache.org/jira/browse/BEAM-12392). Original Jira may contain additional context.
Reported by: ibzib.