Macro bitcoin_internals::write_err

macro_rules! write_err {
    ($writer:expr, $string:literal $(, $args:expr)*; $source:expr) => { ... };
}

Formats error.

If std feature is OFF appends error source (delimited by : ). We do this because e.source() is only available in std builds, without this macro the error source is lost for no-std builds.