fix error propagation in export-env (#14847)

- fixes #14801

# Description

- Fixed the issue
- Added some comments mirroring the ones used in `export-env` handling
in `use`
- Added two tests to prevent regressions

# User-Facing Changes

# Tests + Formatting

- 🟢 toolkit fmt
- 🟢 toolkit clippy
- 🟢 toolkit test
- 🟢 toolkit test stdlib

# After Submitting

crates/nu-command/src/env/export_env.rs

@@ -57,8 +57,10 @@ impl Command for ExportEnv {
 
         let eval_block = get_eval_block(engine_state);
 
-        let _ = eval_block(engine_state, &mut callee_stack, block, input);
+        // Run the block (discard the result)
+        let _ = eval_block(engine_state, &mut callee_stack, block, input)?;
 
+        // Merge the block's environment to the current stack
         redirect_env(engine_state, caller_stack, &callee_stack);
 
         Ok(PipelineData::empty())