make update_progress_bar a bit more robust by just doing nothing if the corresponding progress bar was not started (and making stopping of a non-started progress bar fatal)

[boegel]

Without these changes, calling update_progress_bar when the progress bar was not started yet means trouble:

Traceback (most recent call last):
   ...
  File "/home/kehoste/easybuild-framework/easybuild/tools/output.py", line 304, in update_progress_bar
    (pbar, task_id) = _progress_bar_cache[bar_type]
KeyError: 'extensions'

Raising a KeyError is pretty nasty, since it gives zero info when it's reported without the traceback.

In #3667 I've moved update_progress_bar into EasyBlock.init_ext_instances for example, which sometimes gets called without the start_progress_bar in extensions_step.
In that case, updating the progress bar doesn't make much sense, and making sure it's only called when an extensions progress bar is actually "active" would be tricky (and lead to ugly code).

[branfosj]

Going in, thanks @boegel!