Adjust error wording when loading missing files

```shell
$ SSL_CERT_FILE="notexist" target/debug/examples/google
thread 'main' panicked at examples/google.rs:7:58:
could not load platform certs: Custom { kind: NotFound, error: "could not load certs from dir notexist: No such file or directory (os error 2)" }
```

Note "from dir notexist".  In this case `path.is_file()` is false
(because it doesn't exist) but that doesn't imply it was being
read as a directory.

src/lib.rs

@@ -159,15 +159,15 @@ impl CertPaths {
 
         let mut certs = match &self.file {
             Some(cert_file) => {
-                load_pem_certs(cert_file).map_err(|err| Self::load_err(cert_file, err))?
+                load_pem_certs(cert_file).map_err(|err| Self::load_err(cert_file, "file", err))?
             }
             None => Vec::new(),
         };
 
         if let Some(cert_dir) = &self.dir {
             certs.append(
                 &mut load_pem_certs_from_dir(cert_dir)
-                    .map_err(|err| Self::load_err(cert_dir, err))?,
+                    .map_err(|err| Self::load_err(cert_dir, "dir", err))?,
             );
         }
 
@@ -177,14 +177,10 @@ impl CertPaths {
         Ok(Some(certs))
     }
 
-    fn load_err(path: &Path, err: Error) -> Error {
+    fn load_err(path: &Path, typ: &str, err: Error) -> Error {
         Error::new(
             err.kind(),
-            format!(
-                "could not load certs from {} {}: {err}",
-                if path.is_file() { "file" } else { "dir" },
-                path.display()
-            ),
+            format!("could not load certs from {typ} {}: {err}", path.display()),
         )
     }
 }