* remove std.Build.updateFile. I noticed some people use it from
   build.zig (declare phase) when it is intended only for use in the
   make phase.
   - This also was incorrectly reporting errors with std.log.
 * std.Build.InstallArtifactStep
   - report better errors on failure
   - report whether the step was cached or not
 * std.Build.InstallDirStep: report better error on failure
 * std.Build.InstallFileStep: report better error on failure