bootstrap: a little more explicit error if things are wrong

There seems to be a problem with the way we set the gcc_bootstrap
package build to depend on the bootstrap package with haiku headers.
If said package cannot be built (for example some definitions are
missing for a new architecture), we end up passing an empty string as
the package to use to haikuporter.

The error message given by Haikuporter is confusing, and not easy to
investigate. So, intercept the error earlier to save time for the next
person to hit this problem.

Change-Id: I64f326e5cb3bb0d44632864ad38ad10bb88d0c7b
Reviewed-on: https://review.haiku-os.org/c/1082
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

build/jam/RepositoryRules

@@ -333,6 +333,11 @@ actions BootstrapRepositoryFetchPackage1
 
 	# make Haiku cross devel package path absolute
 	haikuCrossDevelPackage="$(2[1])"
+	if [ "x$haikuCrossDevelPackage" = "x" ]; then
+		echo "$portSpec does not have a cross-devel package defined!"
+		exit 1
+	fi
+
 	if [ "x$haikuCrossDevelPackage" = "x${haikuCrossDevelPackage#/}" ]; then
 		haikuCrossDevelPackage="`pwd`/$haikuCrossDevelPackage"
 	fi