better c interop error checking

src/ls/ls.v

@@ -24,7 +24,7 @@ fn ls(args Args) {
 		if print_dir_names {
 			print_dir_name(dir, args)
 		}
-		
+
 		print_listing(listing, args)
 	}
 }

src/ls/ls_nix.c.v

@@ -1,3 +1,5 @@
+import os
+
 #include <pwd.h>
 #include <grp.h>
 #include <unistd.h>
@@ -21,18 +23,39 @@ fn C.getgrgid(uid usize) &Group
 fn C.readlink(file &char, buf &char, buf_size usize)
 
 fn get_owner_name(uid usize) string {
-	p := C.getpwuid(uid)
-	return unsafe { cstring_to_vstring(p.pw_name) }
+	pwd := C.getpwuid(uid)
+	unsafe {
+		if isnil(pwd) {
+			// Call succeeded but user not found
+			if C.errno == 0 {
+				return ''
+			}
+			return os.error_posix().msg()
+		}
+		return cstring_to_vstring(pwd.pw_name)
+	}
 }
 
 fn get_group_name(uid usize) string {
 	grp := C.getgrgid(uid)
-	return unsafe { cstring_to_vstring(grp.gr_name) }
+	unsafe {
+		if isnil(grp) {
+			// Call succeeded but user not found
+			if C.errno == 0 {
+				return ''
+			}
+			return os.error_posix().msg()
+		}
+		return cstring_to_vstring(grp.gr_name)
+	}
 }
 
 fn read_link(file string) string {
 	buf_size := 2048
 	buf := '\0'.repeat(buf_size)
 	len := C.readlink(file.str, buf.str, usize(buf_size))
+	if len == -1 {
+		return os.error_posix().msg()
+	}
 	return buf.substr(0, len)
 }