Add try/catch error recovery to expr

Normalized profile

What this task asks for

Add expression and block-level error recovery with try, catch, finally, throw, retry, and errtype. Add comprehensive error handling. `try(expression, fallback)` - returns expression result on success or the lazily-evaluated fallback on error; requires exactly two arguments. `try { expr } catch { handler }` - block form; optionally `catch <name> { ... }` to bind the error. `catch <name> is "substring" { ... }` - catches only errors whose message contains the substring. `finally { cleanup }` - optional clause that always executes after try/catch; if the finally body throws, that error propagates (overriding any prior result). `throw(value)` - throws a custom error from any value (the error message is its string conversion); requires exactly one argument.