BuildStream can crash if remote-execution endpoint is not working as expected

Summary

While trying out BuildStream against RBE's remote execution endpoint, I had added the following configuration to doc/examples/autotools/project.conf:

remote-execution:
  url: http://remotebuildexecution.googleapis.com:80

I am pretty sure that this endpoint is probably not going to work without other modifications but still BuildStream should not crash in this case.

Steps to reproduce

• Change directory to doc/examples/autotools
• Append the above config to project.conf
• Run bst build hello.bst
• See BuildStream crash

What is the current bug behavior?

BuildStream crashes with the following

What is the expected correct behavior?

BuildStream crashes like so: https://gitlab.com/snippets/1762252.

Relevant logs and/or screenshots

Possible fixes

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.4

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