better error handling for view source (#14624)

# Description

There is an opportunity to give a bogus block id to view source. This
makes it more resilient and not panic when an invalid block id is passed
in.


![image](https://github.com/user-attachments/assets/67ebbffc-be57-4ce3-8700-90f1ed080f9b)


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crates/nu-command/src/debug/view_source.rs

@@ -34,7 +34,9 @@ impl Command for ViewSource {
 
         let source = match arg {
             Value::Int { val, .. } => {
-                let block = engine_state.get_block(nu_protocol::BlockId::new(val as usize));
+                if let Some(block) =
+                    engine_state.try_get_block(nu_protocol::BlockId::new(val as usize))
+                {
                     if let Some(span) = block.span {
                         let contents = engine_state.get_span_contents(span);
                         Ok(Value::string(String::from_utf8_lossy(contents), call.head)
@@ -48,7 +50,17 @@ impl Command for ViewSource {
                             inner: vec![],
                         })
                     }
+                } else {
+                    Err(ShellError::GenericError {
+                        error: format!("Block Id {} does not exist", arg.coerce_into_string()?),
+                        msg: "this number does not correspond to a block".to_string(),
+                        span: Some(arg_span),
+                        help: None,
+                        inner: vec![],
+                    })
                 }
+            }
+
             Value::String { val, .. } => {
                 if let Some(decl_id) = engine_state.find_decl(val.as_bytes(), &[]) {
                     // arg is a command