Build error with -Werror=return-type

[hhrsscc]

Got a build error with -Werror=return-type enabled

folly/folly/settings/Types.cpp: In function ‘std::string_view folly::settings::toString(folly::settings::SetErrorCode)’:
folly/folly/settings/Types.cpp:31:1: error: control reaches end of non-void function [-Werror=return-type]

[Anxueim]

std::string_view folly::settings::toString(folly::settings::SetErrorCode error)
{
switch (error)
{
case SetErrorCode::kSuccess:
return "kSuccess";
case SetErrorCode::kError:
return "kError";
// add a program
}
// Default return (if program doesn't match)
return "Unknown Error";
}
Function of:  toString(folly::settings::SetErrorCode)  is expected to return a value of type  std::string_view . Therefore, we need to ensure that all execution paths within the function return a value