mobile: be prepared for failure during experiment creation

[bassosimone]

In engine <= 3.18, you can always create an experiment if the experiment name exists. In engine >= 3.19, this is no longer the case, because of ooni/probe-cli#1355. For example, the "echcheck" experiment cannot be created unless the check-in API has authorized us to do so in the last 24h. The cli code is already handling this case correctly and there seems to be no database side effect. We need to make sure the same holds for the mobile apps.

[aanorbel]

Supported as part of ooni/probe-android#622 .
This also introduces a situation where the test suite can be successful and still has a list of errors.
See the example below for a sample experimental test suite.

echcheck: experiment not enabled by check-in API

torsf: experiment not enabled by check-in API

riseupvpn: experiment not enabled by check-in API

I propose we come up with a way to inform the users that some experiments were not started.
We have had issues reported of experiments not being run when they are enabled in the test options #2204

cc: @bassosimone , @agrabeli , @hellais

[bassosimone]

@aanorbel the engine communicates errors to the mobile apps using strings. We can use the "experiment not enabled by check-in API" string to know that an experiment was disabled by a OONI backend policy. If we choose to go down this road, please, let me know, so that I can document the specific error string as precious for the apps and thus increase the probability that it would not be changed by mistake when refactoring.

[aanorbel]

Since there is currently no way to inform users about the OONI backend policy disabling a specific test, I propose we discuss it in the next front-end meeting so a decision can be taken.