Add partial support for Erlang/OTP 17

Allow rebar to compile applications using Erlang/OTP 17 and older
versions.  This patch only provides partial support since the rebar
tool itself must be compiled using an Erlang/OTP version that is older
than 17.
Merge pull request #237 from norton/norton-17

Add partial support for Erlang/OTP 17
file_utils: properly report errors (fix #95)

While at it, improve the error message printed by rebar_utils:sh/2.