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/importcycle/cycle.go

@@ -0,0 +1,3 @@
+package importcycle
+
+import _ "github.com/tinygo-org/tinygo/testdata/errors/importcycle"

testdata/errors/invaliddep/invaliddep.go

@@ -0,0 +1 @@
+ppackage // syntax error

testdata/errors/loader-importcycle.go

@@ -0,0 +1,10 @@
+package main
+
+import _ "github.com/tinygo-org/tinygo/testdata/errors/importcycle"
+
+func main() {
+}
+
+// ERROR: package command-line-arguments
+// ERROR: 	imports github.com/tinygo-org/tinygo/testdata/errors/importcycle
+// ERROR: 	imports github.com/tinygo-org/tinygo/testdata/errors/importcycle: import cycle not allowed

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

testdata/errors/loader-invalidpackage.go

@@ -0,0 +1,3 @@
+ppackage // syntax error
+
+// ERROR: loader-invalidpackage.go:1:1: expected 'package', found ppackage

testdata/errors/loader-nopackage.go

@@ -0,0 +1,14 @@
+package main
+
+import (
+	_ "github.com/tinygo-org/tinygo/testdata/errors/non-existing-package"
+	_ "github.com/tinygo-org/tinygo/testdata/errors/non-existing-package-2"
+)
+
+func main() {
+}
+
+// ERROR: loader-nopackage.go:4:2: no required module provides package github.com/tinygo-org/tinygo/testdata/errors/non-existing-package; to add it:
+// ERROR: 	go get github.com/tinygo-org/tinygo/testdata/errors/non-existing-package
+// ERROR: loader-nopackage.go:5:2: no required module provides package github.com/tinygo-org/tinygo/testdata/errors/non-existing-package-2; to add it:
+// ERROR: 	go get github.com/tinygo-org/tinygo/testdata/errors/non-existing-package-2