Do not hide errors from e.g. 7-zip

[jebej]

Not sure if this should go in the main julia repo or not.

I ran into an issue where the "zlib1" installation failed as part of a package building process, but the relevant error message was not shown.

INFO: Packages to update:  zlib1, libwinpthread1
INFO: Packages to install: libmpc3, libgmp10, libmpfr4, gcc-c++
INFO: Downloading: zlib1
INFO: Extracting: zlib1
===================================================[ ERROR: Clipper ]===================================================

LoadError: failed process: Process(`'C:\Users\Jeremy\AppData\Local\Julia-0.6.0\bin\7z.exe' x -y 'C:\Users\Jeremy\.julia\v0.6\WinRPM\cache\4\noarch%2Fmingw64-zlib1-1.2.8-8.6.noarch.cpio' '-oC:\Users\Jeremy\.julia\v0.6\WinRPM\deps'`, ProcessExited(2)) [2]
while loading C:\Users\Jeremy\.julia\v0.6\Clipper\deps\build.jl, in expression starting on line 12

========================================================================================================================

If instead I install the lib manually, the error is shown, which helps to resolve the error:

julia> WinRPM.install("zlib1")
INFO: Packages to update:  zlib1
Continue with updates [y/N]? y
INFO: Downloading: zlib1
INFO: Extracting: zlib1

7-Zip [64] 9.20  Copyright (c) 1999-2010 Igor Pavlov  2010-11-18

Processing archive: C:\Users\Jeremy\.julia\v0.6\WinRPM\cache\4\noarch%2Fmingw64-zlib1-1.2.8-8.6.noarch.cpio

ERROR: Can not delete output file C:\Users\Jeremy\.julia\v0.6\WinRPM\deps\usr\x86_64-w64-mingw32\sys-root\mingw\bin\zlib1.dll

Sub items Errors: 1


ERROR: failed process: Process(`'C:\Users\Jeremy\AppData\Local\Julia-0.6.0\bin\7z.exe' x -y 'C:\Users\Jeremy\.julia\v0.6\WinRPM\ca
che\4\noarch%2Fmingw64-zlib1-1.2.8-8.6.noarch.cpio' '-oC:\Users\Jeremy\.julia\v0.6\WinRPM\deps'`, ProcessExited(2)) [2]

[musm]

Good suggestion