Use std::runtime_error instead of our own StringError as

Use std::runtime_error instead of our own StringError as
a) it does the same job and
b) its type and what() survive the boundary between the libdcp .so
and the main DCP-o-matic executable.

Before this StringError-derived exceptions caught by DCP-o-matic
were only recognised as std::exceptions (without the what()) message.

I don't know why this happens, but this works around it.