Add comments to the No*InContext error paths

Signed-off-by: Jacinta Ferrant <jacinta@stackslabs.com>

Author: jacinta-stacks

clarity/src/vm/variables.rs

@@ -58,10 +58,14 @@ pub fn lookup_reserved_variable(
     {
         match variable {
             NativeVariables::TxSender => {
+                // This `NoSenderInContext` is **unreachable** in standard Clarity VM execution.
+                // - Every function call (public, private, or trait) is executed with a valid caller context.
                 let sender = env.sender.clone().ok_or(RuntimeError::NoSenderInContext)?;
                 Ok(Some(Value::Principal(sender)))
             }
             NativeVariables::ContractCaller => {
+                // This `NoCallerInContext` is **unreachable** in standard Clarity VM execution.
+                // -  Every on-chain transaction and contract-call has a well-defined sender.
                 let caller = env.caller.clone().ok_or(RuntimeError::NoCallerInContext)?;
                 Ok(Some(Value::Principal(caller)))
             }