Show a proper error message if the build exceeded the number of retries

[stsewd]

Currently if the number of retries is reached (25)

readthedocs.org/readthedocs/settings/base.py

Line 121 in 7db6582

RTD_BUILDS_MAX_RETRIES = 25

An exception will be raised (BuildMaxConcurrencyError), and we will be hitting this code

readthedocs.org/readthedocs/projects/tasks/builds.py

Lines 416 to 418 in ea5d9e3

	elif isinstance(exc, BuildUserError):
	if hasattr(exc, 'message') and exc.message is not None:
	self.data.build['error'] = exc.message

that code just updates the build to the same error as Concurrency limit reached (2), retrying in 5 minutes, and mark the build as failed, that isn't accurate as the build will not be retried.

[humitos]

I think this is a dup of #9011