This change was missing in 47fa8c4c

The scan methods must be "noexcept".

The CurlRequest constructor can throw exceptions, therefore we must
not use "noexcept".

Allows catchers to inspect the HTTP status.

See commit be137a19

Since 7d353bbe, _GNU_SOURCE is always
defined, which implies __USE_MINGW_ANSI_STDIO and thus switches to
the mingw implementations of the printf() family.  That's
standards-compliant, unlike Microsoft's CRT implementations.

Closes #203

Eliminate duplicate code.

Closes #202