Duplicated error messages across multiple files

[jirispilka]

Create src/errors.ts with error functions (following apify-mcp-server-internal pattern):

Functions returning McpError for thrown errors
Functions returning buildMCPResponse() for returned errors

Files: Create src/errors.ts, update actor.ts, fetch-actor-details.ts, fetch-apify-docs.ts, server.ts, store_collection.ts

Examples: actorNotFound(), mcpServerConnectionFailed(), docsFetchFailed(), toolNotFound()

[jirispilka]

Closed in: #335