Improve some errors, streamline internal error handling (#2839)

* Improve some errors, streamline internal error handling

* Fix lints

crates/nu-cli/tests/commands/enter.rs

@@ -80,6 +80,6 @@ fn errors_if_file_not_found() {
             "enter i_dont_exist.csv"
         );
 
-        assert!(actual.err.contains("Cannot canonicalize"));
+        assert!(actual.err.contains("Cannot find file"));
     })
 }

crates/nu-cli/tests/commands/open.rs

@@ -222,7 +222,7 @@ fn errors_if_file_not_found() {
         cwd: "tests/fixtures/formats",
         "open i_dont_exist.txt"
     );
-    let expected = "Cannot canonicalize";
+    let expected = "Cannot find file";
     assert!(
         actual.err.contains(expected),
         "Error:\n{}\ndoes not contain{}",