compiler: fix panic on import errors

I think this is a problem in the loader package - it doesn't seem to
return errors for missing packages.

compiler/compiler.go

@@ -166,8 +166,7 @@ func (c *Compiler) Parse(mainPath string) error {
 			Compiler:    "gc", // must be one of the recognized compilers
 			BuildTags:   append([]string{"tgo"}, c.BuildTags...),
 		},
-		ParserMode:  parser.ParseComments,
-		AllowErrors: true,
+		ParserMode: parser.ParseComments,
 	}
 	config.Import("runtime")
 	config.Import(mainPath)
@@ -176,13 +175,6 @@ func (c *Compiler) Parse(mainPath string) error {
 		return err
 	}
 
-	// TODO: pick the error of the first package, not a random package
-	for _, pkgInfo := range lprogram.AllPackages {
-		if len(pkgInfo.Errors) != 0 {
-			return pkgInfo.Errors[0]
-		}
-	}
-
 	c.ir = ir.NewProgram(lprogram, mainPath)
 
 	// Make a list of packages in import order.