verbose backtraces when “invoke” fails

[Ricardo Wurmus]

Hi Guix,

I’d like us to catch the errors raised by “invoke”, so that the output
of a failing build looks nicer again.

Before we used “invoke” build phases evaluating to #f would cause the
build system to abort and print a nice message.  Now users get an ugly
backtrace that is much too verbose for what it’s trying to say, namely
that a command invocation failed.

There is no value in the backtrace itself (users don’t care where
“invoke” is defined, for example); we only want the error message.
Shall we fix this on core-updates?

--
Ricardo