Sinking an unsuccessful Proc should throw.

This means `run` anad `shell` in sink context will not fail silently
if they are not successful.

src/core/Exception.pm

@@ -2272,4 +2272,11 @@ my class X::Language::Unsupported is Exception {
     }
 }
 
+my class X::Proc::Unsuccessful is Exception {
+    has $.proc;
+    method message() {
+        "The spawned process exited unsuccessfully (exit code: $.proc.exitcode())"
+    }
+}
+
 # vim: ft=perl6 expandtab sw=4

src/core/Proc.pm

@@ -125,6 +125,10 @@ my class Proc {
     multi method status(Proc:D:)  { ($!exitcode +< 8) +| $!signal }
     multi method Numeric(Proc:D:) { $!exitcode }
     multi method Bool(Proc:D:)    { $!exitcode == 0 }
+
+    method sink(--> Nil) {
+        X::Proc::Unsuccessful.new(:proc(self)).throw unless self;
+    }
 }
 
 sub run(*@args ($, *@), :$in = '-', :$out = '-', :$err = '-',