Raise error when filename has spaces

[parham-k]

Addresses #129

Attempted to fix by:

• putting the path in quotes interprets input as two files with one starting with quotes and the other ending with one.
• adding a backslash before each space, similar to quotes, interprets one file ending with \
• using std::filesystem::exists instead of stat and std::filesystem::path(source_path).u8string(), no change

Added a check_error for this case to provide user with a verbose error message.