[fix] METEOR GUI: handle better incorrect z-stack parameters

Currently, if the user enters incorrect z-stack paramers (eg, a very
small zstep, causing > 500 steps), no indication is shown, until they
start an acquisition, and then the error is shown in the log, while the
GUI stays entirely disabled (because is_acquiring is still set to True).

Change so that:
  * An error message is shown instead of the (incorrect) time estimation
  * The acquire buttons cannot be pressed
  * Even if somehow the user manages to start the acquisition, the GUI is
    not disabled.

Author: pieleric

src/odemis/gui/cont/acquisition/cryo_acq.py

@@ -134,6 +134,9 @@ def __init__(self, tab_data, panel, tab, mode: guimod.AcquiMode):
         self._panel.streams_chk_list.Bind(wx.EVT_CHECKLISTBOX, self._on_check_list)
 
         self._zlevels: Dict[Stream, List[float]] = {}
+        # When zstack_error is set to a string, it contains the error message to show, explaining
+        # why a z-stack cannot be done.
+        self._zstack_error: Optional[str] = None
 
         # common VA's
         self._tab_data.filename.subscribe(self._on_filename, init=True)
@@ -653,6 +656,7 @@ def _update_acquisition_time(self):
             return
 
         # estimate the total time for acquiring at features
+        can_acquire = not self._tab_data.main.is_acquiring.value
         features = self._get_selected_features()
         if not features:
             txt = "No features selected."