loader: handle go list errors inside TinyGo

Instead of exiting with an error, handle these errors internally.
This will enable a few improvements in the future.

testdata/errors/loader-invaliddep.go

@@ -0,0 +1,8 @@
+package main
+
+import _ "github.com/tinygo-org/tinygo/testdata/errors/invaliddep"
+
+func main() {
+}
+
+// ERROR: invaliddep/invaliddep.go:1:1: expected 'package', found ppackage