Fix use of errno in SystemError

[Bo98]

Calling errno inside SystemError.description() is far too late and will almost always return the wrong error number. Instead, pass the errno at the throw site, which is consistent with how most of the other SystemError throws work.

[compnerd]

This seems like the right thing to do. Should double check that there's no other instance that needs to be updated for close.

[compnerd]

@swift-ci please test

[MaxDesiatov]

I wonder if there's a test case one could come up with to reproduce the original issue, verify that the change fixes it, and prevent future regressions?

[neonichu]

I think we should continue to check for a return value of -1 before we consult errno.

[Bo98]

I think we should continue to check for a return value of -1 before we consult errno.

Only return values of 0 and -1 are defined in POSIX. So what value we should be using instead for a theoretical -2 is unclear to me.

RETURN VALUES
     Upon successful completion, a value of 0 is returned.  Otherwise, a value
     of -1 is returned and the global integer variable errno is set to
     indicate the error.

[jakepetroules]

@swift-ci test