Mount: remove broad HandleRequest exception catch

[tyrielv]

Summary

Follow-up to PR #2021 (merged). Changes the outer try-catch in HandleRequest from swallowing exceptions to catch-log-rethrow, so fail-fast via Environment.Exit is preserved while adding diagnostic context.

Problem

The original broad catch suppressed all exceptions (except OutOfMemoryException) from every pipe request handler. This could silently mask corruption in state-mutating handlers:

• PostIndexChanged — projection update mid-flight → stale/wrong projection
• DehydrateFolders — folder moves + reset → half-dehydrated state
• ReleaseLock — lock bookkeeping corruption

Instead of crashing (which surfaces the problem immediately), the mount would continue serving in an inconsistent state.

Fix

The catch now logs the message header and exception details (context that OnNewConnection's generic LogErrorAndExit in NamedPipeServer does not capture), then rethrows so the exception propagates to OnNewConnection → Environment.Exit as before.

What stays unchanged

The inner try-catch in HandleDownloadObjectRequest remains — it covers the specific crash scenario #2021 addressed. The download path is safe to catch and suppress:

• Loose object writes are atomic (temp file → rename)
• missingTreeTracker is advisory (re-populated on next request)
• No critical mount state (projection, lock, placeholder DB) is mutated
• Returns proper DownloadFailed response instead of broken pipe

Testing

• Unit tests: 876 passed, 0 failed
• The inner DownloadObject catch continues to prevent the flaky rebase test crash