Improve error handling with issueable blocking quickactions

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Problem to solve

The /blocked_by and /blocks quick actions for work item dependencies for both Issue and MergeRequest currently lack reference validation as discussed in !207136 (comment 3114431096).

When users provide invalid or non-existent work item references, the commands fail silently without providing feedback.

Proposal

We need to ensure the passed-in references are valid. If not, we need to return an error message to communicate that to the user. Since we may have multiple references, we need to show successful messages and failed messages together if it fails partially.

Example scenarios:

• /blocks !123 !456 !789 where !456 doesn't exist -> Show success for !123 and !789, error for !456
• /blocked_by !100 !200 where both are invalid -> Show errors for both references
• /blocks !123 where !123 is valid -> Show success message only