Corpus Viewer: Sanitize bad regex

[ajdapretnar]

Describe the bug
Corpus Viewer crashes on bad regex.

------------------------------- error Exception -------------------------------
Traceback (most recent call last):
  File "/Applications/Orange.app/Contents/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/orangewidget/gui.py", line 993, in returnPressedHandler
    self.__callback_if_changed()
  File "/Applications/Orange.app/Contents/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/orangewidget/gui.py", line 1001, in __callback_if_changed
    self.callback()
  File "/Applications/Orange.app/Contents/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/orangecontrib/text/widgets/owcorpusviewer.py", line 591, in refresh_search
    self.doc_list.model().set_filter_string(self.regexp_filter)
  File "/Applications/Orange.app/Contents/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/orangecontrib/text/widgets/owcorpusviewer.py", line 189, in set_filter_string
    self.__regex = re.compile(filter_string.strip("|"), re.IGNORECASE)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Applications/Orange.app/Contents/Frameworks/Python.framework/Versions/3.11/lib/python3.11/re/__init__.py", line 227, in compile
    return _compile(pattern, flags)
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Applications/Orange.app/Contents/Frameworks/Python.framework/Versions/3.11/lib/python3.11/re/__init__.py", line 294, in _compile
    p = _compiler.compile(pattern, flags)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Applications/Orange.app/Contents/Frameworks/Python.framework/Versions/3.11/lib/python3.11/re/_compiler.py", line 745, in compile
    p = _parser.parse(p, flags)
        ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Applications/Orange.app/Contents/Frameworks/Python.framework/Versions/3.11/lib/python3.11/re/_parser.py", line 989, in parse
    p = _parse_sub(source, state, flags & SRE_FLAG_VERBOSE, 0)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Applications/Orange.app/Contents/Frameworks/Python.framework/Versions/3.11/lib/python3.11/re/_parser.py", line 464, in _parse_sub
    itemsappend(_parse(source, state, verbose, nested + 1,
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Applications/Orange.app/Contents/Frameworks/Python.framework/Versions/3.11/lib/python3.11/re/_parser.py", line 548, in _parse
    code = _escape(source, this, state)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Applications/Orange.app/Contents/Frameworks/Python.framework/Versions/3.11/lib/python3.11/re/_parser.py", line 396, in _escape
    raise source.error("incomplete escape %s" % escape, len(escape))
re.error: incomplete escape \u at position 0
-------------------------------------------------------------------------------

To Reproduce
Open Corpus Viewer with any corpus, enter \u in the search box, press Enter.

Expected behavior
A Warning occurs.

Orange version:
3.37.0

Text add-on version:
1.15.0

Operating system:
OSX