Remove an unused error code

I don't see this error code being useful - working slowly hardly
would be a reason for the library to return and fail to do what
it was asked to do, so normal functions would never actually return
this code.
disable most warnings produced by -Wall
fix most of the warnings
Merge pull request #541 from licaiguo/disable-warnings

disable most warnings produced by -Wall
Merge pull request #529 from mstorsjo/remove-unused-error-code

Remove an unused error code
Merge remote-tracking branch 'upstream/master' into travis.android