UploadPack: Introduce ErrorWriter

UploadPack: Introduce ErrorWriter

ErrorWriter writes an error message to the user. The implementation is
swapped once it detects that the client supports sideband. By default it
uses the protocol level ERR packet, which was introduced recently.

In total the error output is done in two different places;
UploadPack#upload and UploadPack#sendPack. These will be consolidated in
the next change.

Change-Id: Ia8d72e31170bbeafc8ffa8ddb92702196af8a587
Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>