Gracefully handle errors when loading themes (zed-industries#6904)

This PR improves the theme loading to gracefully handle when a theme
couldn't be loaded.

Trying to debug a panic that happens on startup in Nightly.

Release Notes:

- N/A

zed 0.120.3

zed 0.119.20

collab 0.42.0

zed 0.120.2

zed 0.119.19

collab 0.41.0

zed 0.120.1

Revert "Ensure that notify observations are sent during `Window::draw…

…()`" (zed-industries#6152)

Reverts zed-industries#4236

This causes an infinite loop when opening the language server logs

zed 0.119.18