Update error message to forward platform-specific error

Author: sethmlarson

src/pip/_internal/cli/req_command.py

@@ -59,10 +59,9 @@ def _create_truststore_ssl_context() -> Optional["SSLContext"]:
 
     try:
         from pip._vendor import truststore
-    except ImportError:
+    except ImportError as e:
         raise CommandError(
-            "To use the truststore feature, 'truststore' must be installed into "
-            "pip's current environment."
+            f"The truststore feature is unavailable: {e}"
         )
 
     return truststore.SSLContext(ssl.PROTOCOL_TLS_CLIENT)