Reset plugin state on crash in extension.snippet.render()

[zoni]

Prior to this change, an exception in render would make the plugin hang on "Loading" when an exception like the following happens:

ulauncher-snippets | 2021-05-07 14:52:08,908 | DEBUG | ulauncher.api.client.Client: on_message() | Incoming event ItemEnterEvent
Traceback (most recent call last):
  File "/home/zoni/Workspace/zoni/ulauncher-snippets/main.py", line 57, in on_event
    snippet = extension.snippet.render(copy_mode=copy_mode)
  File "/home/zoni/Workspace/zoni/ulauncher-snippets/src/functions.py", line 103, in render
    template = Template(snippet.content)
  File "/usr/lib/python3.9/site-packages/jinja2/environment.py", line 1031, in __new__
    return env.from_string(source, template_class=cls)
  File "/usr/lib/python3.9/site-packages/jinja2/environment.py", line 941, in from_string
    return cls.from_code(self, self.compile(source), globals, None)
  File "/usr/lib/python3.9/site-packages/jinja2/environment.py", line 638, in compile
    self.handle_exception(source=source_hint)
  File "/usr/lib/python3.9/site-packages/jinja2/environment.py", line 832, in handle_exception
    reraise(*rewrite_traceback_stack(source=source))
  File "/usr/lib/python3.9/site-packages/jinja2/_compat.py", line 28, in reraise
    raise value.with_traceback(tb)
  File "<unknown>", line 1, in template
jinja2.exceptions.TemplateAssertionError: no filter named 'replace_with_symbol'

After closing the ulauncher window and then attempting to insert a new
snippet, it would hang without feedback because of corruption of its
internal state:

ulauncher-snippets | 2021-05-07 14:52:15,427 | DEBUG | ulauncher.api.client.Client: on_message() | Incoming event KeywordQueryEvent
Traceback (most recent call last):
  File "/usr/lib/python3.9/site-packages/ulauncher/api/client/Client.py", line 54, in on_message
    self.extension.trigger_event(event)
  File "/usr/lib/python3.9/site-packages/ulauncher/api/client/Extension.py", line 52, in trigger_event
    action = listener.on_event(event, self)
  File "/home/zoni/Workspace/zoni/ulauncher-snippets/main.py", line 82, in on_event
    show_var_input(extension.snippet, extension.variable, search)
AttributeError: 'SnippetsExtension' object has no attribute 'variable'

With this change, this code path is wrapped in try/except which will (1) display the exception message to the end-user and (2) consistently reset plugin state afterwards.

[mikebarkmin]

lgtm 🎆