create-pull-request: Improved the error-handling
create-pull-request: Improved the error-handling

The error handling is improved to warn the user about errors
in "git format-patch" and in case the user supplied a changeset
without any diffs.

Signed-off-by: Esben Haabendal <c5ba3f1141bb30a64dd6c7e6726a53a85adc9442@prevas.dk>

hundeboll/core,sknsean/core,sknsean/core,sknsean/core,sknsean/core,hundeboll/core,hundeboll/core,hundeboll/core,diorcety/oe-lite-core,sknsean/core,diorcety/oe-lite-core