Fix error message when importing package that does not exist

Previously on Linux, if a file in your program tried to import a
package that did not actually exist, read_directory() assumed that
the errno after calling opendir() was ENOTDIR.

This was incorrect.

Instead, we now switch on errno and check for ENOENT, which it is
if the directory does not exist.

src/common.cpp

@@ -968,7 +968,20 @@ ReadDirectoryError read_directory(String path, Array<FileInfo> *fi) {
 
 	DIR *dir = opendir(c_path);
 	if (!dir) {
-		return ReadDirectory_NotDir;
+		switch (errno) {
+		case ENOENT:
+			return ReadDirectory_NotExists;
+		case EACCES:
+			return ReadDirectory_Permission;
+		case ENOTDIR:
+			return ReadDirectory_NotDir;
+		default:
+			// ENOMEM: out of memory
+			// EMFILE: per-process limit on open fds reached
+			// ENFILE: system-wide limit on total open files reached
+			return ReadDirectory_Unknown;
+		}
+		GB_PANIC("unreachable");
 	}
 
 	array_init(fi, a, 0, 100);