[Discover] Render the sidebar even if documents fetching failed

[jughosta]

This is a follow up PR for #144412 to fix how "Error" case is handled.

Before: the sidebar was stuck in loading state:

After: the sidebar is rendered:

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 8cc0d96

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
discover	418.2KB	418.3KB	+112.0B

Unknown metric groups

ESLint disabled in files

id	before	after	diff
osquery	1	2	+1

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	19	21	+2
fleet	59	65	+6
osquery	109	115	+6
securitySolution	445	451	+6
total			+20

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	20	22	+2
fleet	68	74	+6
osquery	110	117	+7
securitySolution	521	527	+6
total			+21

History

• 💚 Build #93302 succeeded 2405b5f
• 💚 Build #93279 succeeded 47b675b

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @jughosta

[davismcphee]

Pulled and tested locally + ran functional test. LGTM, thanks for fixing!