Move definition of functions declared in error.h to error.cpp
* src/CMakeLists.txt (Gpgmepp_SOURCES): Add new file. * src/context.cpp (format_error, Error::source, Error::asString, Error::asStdString, Error::code, Error::sourceID, Error::isSuccess, Error::isCanceled, Error::isError, Error::toErrno, Error::hasSystemError, Error::setSystemError, Error::setErrno, Error::fromSystemError, Error::fromErrno, Error::fromCode, operator<<(std::ostream &, const Error &)): Move to... * src/error.cpp: ...this new file.