Random 502 in conan proxy?

```
2020-10-09 12:32:20,137 lib.installation INFO Building catch2 for [g484,,/opt/compiler-explorer/gcc-4.8.4,Linux,Debug,x86,,,]
Traceback (most recent call last):
File "", line 1, in
File "/tmp/build/infra/bin/lib/ce_install.py", line 220, in main
raise e
File "/tmp/build/infra/bin/lib/ce_install.py", line 213, in main
[num_installed, num_skipped, num_failed] = installable.build(args.buildfor)
File "/tmp/build/infra/bin/lib/installation.py", line 399, in build
return builder.makebuild(buildfor)
File "/tmp/build/infra/bin/lib/library_builder.py", line 770, in makebuild
buildstatus = self.makebuildfor(compiler, options, exe, compilerType, toolchain, buildos, buildtype, arch, stdver, stdlib, flagscombination)
File "/tmp/build/infra/bin/lib/library_builder.py", line 621, in makebuildfor
if self.has_failed_before() and not self.forcebuild:
File "/tmp/build/infra/bin/lib/library_builder.py", line 545, in has_failed_before
raise RuntimeError(f'Post failure for {url}: {request}')
RuntimeError: Post failure for https://conan.compiler-explorer.com/hasfailedbefore:
Traceback (most recent call last):
File "", line 1, in
File "/home/ubuntu/infra/bin/lib/ce.py", line 932, in main
globals()[cmd + "_cmd"](kwargs)
File "/home/ubuntu/infra/bin/lib/ce.py", line 221, in builder_cmd
dispatch_global('builder', args)
File "/home/ubuntu/infra/bin/lib/ce.py", line 36, in dispatch_global
globals()['{}_{}_cmd'.format(sub, args['{}_sub'.format(sub)])](args)
File "/home/ubuntu/infra/bin/lib/ce.py", line 231, in builder_exec_cmd
exec_remote_to_stdout(instance, args['remote_cmd'])
File "/home/ubuntu/infra/bin/lib/ssh.py", line 60, in exec_remote_to_stdout
subprocess.check_call(ssh_args_for(command, instance))
File "/usr/lib/python3.8/subprocess.py", line 364, in check_call
raise CalledProcessError(retcode, cmd)
```

Call should also probably be a GET in this case, there's no need to login for this command.