Only register sys.excepthook if the app actually started

Without this patch, when running tests the crash message box may be displayed.
With those changes, it will be displayed only if the app was actually started.

picard/__init__.py

@@ -132,10 +132,10 @@ def crash_handler(exc: Exception = None):
     app.quit()
 
 
-def _global_exception_handler(exctype, value, traceback):
-    from picard import crash_handler
-    crash_handler(exc=value)
-    sys.__excepthook__(exctype, value, traceback)
+def register_excepthook():
+    def _global_exception_handler(exctype, value, traceback):
+        from picard import crash_handler
+        crash_handler(exc=value)
+        sys.__excepthook__(exctype, value, traceback)
 
-
-sys.excepthook = _global_exception_handler
+    sys.excepthook = _global_exception_handler

picard/tagger.py

@@ -73,6 +73,7 @@ from picard import (
     PICARD_ORG_NAME,
     acoustid,
     log,
+    register_excepthook,
 )
 from picard.acoustid.manager import AcoustIDManager
 from picard.album import (
@@ -1505,6 +1506,8 @@ If a new instance will not be spawned files/directories will be passed to the ex
 
 
 def main(localedir=None, autoupdate=True):
+    register_excepthook()
+
     # Some libs (ie. Phonon) require those to be set
     QtWidgets.QApplication.setApplicationName(PICARD_APP_NAME)
     QtWidgets.QApplication.setOrganizationName(PICARD_ORG_NAME)