transform: replace panics with source locations

Panics are bad for usability: whenever something breaks, the user is
shown a (not very informative) backtrace. Replace it with real error
messages instead, that even try to display the Go source location.

transform/interface-lowering_test.go

@@ -2,9 +2,16 @@ package transform
 
 import (
 	"testing"
+
+	"tinygo.org/x/go-llvm"
 )
 
 func TestInterfaceLowering(t *testing.T) {
 	t.Parallel()
-	testTransform(t, "testdata/interface", LowerInterfaces)
+	testTransform(t, "testdata/interface", func(mod llvm.Module) {
+		err := LowerInterfaces(mod)
+		if err != nil {
+			t.Error(err)
+		}
+	})
 }