Be more discriminating in failure popups

[jmcphers]

This change prevents superfluous error notifications/toasts when running cells in Positron.

The issue was that we were not catching exceptions thrown by executeCode. It is appropriate for exceptions to propagate here when we can't execute the code for some structural reason, but most often the failure is caused by the code itself.

When a cell fails to execute (a runtime/syntax error, an interrupt, or the session exiting) Positron already reports it in the console. Previously the rejection from executeCode also propagated up to the command handler, which surfaced it a second time as a notification popup.

This change distinguishes the two failure modes via the execution observer's onFailed callback:

• Runtime-reported failures (the code ran and errored): logged at debug level and swallowed so they aren't shown twice. Subsequent blocks are skipped since one failed.
• Submission failures (e.g. no runtime registered for the language): logged as an error and re-thrown so the user is notified, since these aren't otherwise surfaced.

Fixes posit-dev/positron#9845

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