message: improv: fix error message
diff: @@ -116,8 +116,7 @@ impl<'b> Shell<'b> {
  }
  if !Variables::is_valid_name(key.name) {
  - return Err("invalid variable name\nVariable names may only be (unicode) \
  - alphanumeric or `_`\nThe first character must be alphabetic or `_`"
  + return Err("invalid variable name: only alphanumerical characters and underscores are supported"
  .to_string());
  }
  