Specify which file not found in error (#11868)

# Description
Currently, `ShellError::FileNotFound` shows the span where the error
occurred but doesn't say which file wasn't found. This PR makes it so
the help includes that (like the `DirectoryNotFound` error).

# User-Facing Changes
No breaking changes, it's just that when a file can't be found, the help
will say which file couldn't be found:


![image](https://github.com/nushell/nushell/assets/45539777/e52f1e65-55c1-4cd2-8108-a4ccc334a66f)

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

@@ -56,6 +56,7 @@ impl Command for SourceEnv {
             PathBuf::from(&path)
         } else {
             return Err(ShellError::FileNotFound {
+                file: source_filename.item,
                 span: source_filename.span,
             });
         };

crates/nu-command/src/filesystem/mv.rs

@@ -81,6 +81,7 @@ impl Command for Mv {
 
         if sources.is_empty() {
             return Err(ShellError::FileNotFound {
+                file: spanned_source.item.to_string(),
                 span: spanned_source.span,
             });
         }

crates/nu-command/src/filesystem/open.rs

@@ -92,7 +92,10 @@ impl Command for Open {
 
             for path in nu_engine::glob_from(&path, &cwd, call_span, None)
                 .map_err(|err| match err {
-                    ShellError::DirectoryNotFound { span, .. } => ShellError::FileNotFound { span },
+                    ShellError::DirectoryNotFound { span, .. } => ShellError::FileNotFound {
+                        file: path.item.to_string(),
+                        span,
+                    },
                     _ => err,
                 })?
                 .1

crates/nu-command/src/filesystem/ucp.rs

@@ -205,7 +205,10 @@ impl Command for UCp {
                     .map(|f| f.1)?
                     .collect();
             if exp_files.is_empty() {
-                return Err(ShellError::FileNotFound { span: p.span });
+                return Err(ShellError::FileNotFound {
+                    file: p.item.to_string(),
+                    span: p.span,
+                });
             };
             let mut app_vals: Vec<PathBuf> = Vec::new();
             for v in exp_files {

crates/nu-command/src/filesystem/umv.rs

@@ -121,7 +121,10 @@ impl Command for UMv {
                     .map(|f| f.1)?
                     .collect();
             if exp_files.is_empty() {
-                return Err(ShellError::FileNotFound { span: p.span });
+                return Err(ShellError::FileNotFound {
+                    file: p.item.to_string(),
+                    span: p.span,
+                });
             };
             let mut app_vals: Vec<PathBuf> = Vec::new();
             for v in exp_files {

crates/nu-command/src/system/nu_check.rs

@@ -120,6 +120,7 @@ impl Command for NuCheck {
                                 path
                             } else {
                                 return Err(ShellError::FileNotFound {
+                                    file: path_str.item,
                                     span: path_str.span,
                                 });
                             }