Handle NameError during profiler initialization

[ivoanjo]

In #1545 a user reported a NameError during profiler initialization.

I wasn't able to reproduce the issue, but I've added a small check and a nicer message in case it comes back to ask users to report the issue to us, and that avoids breaking their application.

Issue #1545

[ericmustin]

feel free to disregard my comment, just a suggestion, otherwise lgtm

[ivoanjo]

Update: Another customer ran into this issue a few weeks back and got the "Profiling was marked as supported and enabled, but setup task was not loaded properly." that we added in this PR.

This is what happened:

• Rails app
• Customer was not running their Ruby app with bundle exec
• When the profiler loaded in ddtracerb exec, the path to load google-protobuf was not setup correctly, so the profiler was not required because Datadog::Profiling.supported? returned false.
• Then, Rails initialized bundler correctly during its initialization
• Later, the customer called Datadog.configure { ... } which re-triggered starting the profiler, and now Datadog::Profiling.supported? returned true, but the profiler had not properly being required so this issue was triggered.

I'll create an issue to improve the user experience around this issue.