Handle EROFS errors

A EROFS will be raised when trying to write a dir onto a
read-only filesystem.  However, if the dir already is there,
then it should be treated like an EEXIST (since EROFS can
be raised by trying to create a dir over a mount point,
where the mount point is not read-only, but you still can't
just clobber over it).

If the dir doesn't already exist, then the EROFS will be
accurately reported as the reason why it could not be
created.

index.js

@@ -9,7 +9,7 @@ function mkdirP (p, mode, f, made) {
         mode = 0777 & (~process.umask());
     }
     if (!made) made = null;
-    
+
     var cb = f || function () {};
     if (typeof mode === 'string') mode = parseInt(mode, 8);
     p = path.resolve(p);
@@ -27,6 +27,11 @@ function mkdirP (p, mode, f, made) {
                 });
                 break;
 
+            case 'EROFS':
+                // a read-only file system.
+                // However, the dir could already exist, in which case
+                // the EROFS error will be obscuring a EEXIST!
+                // Fallthrough to that case.
             case 'EEXIST':
                 fs.stat(p, function (er2, stat) {
                     // if the stat fails, then that's super weird.
@@ -48,7 +53,7 @@ mkdirP.sync = function sync (p, mode, made) {
         mode = 0777 & (~process.umask());
     }
     if (!made) made = null;
-    
+
     if (typeof mode === 'string') mode = parseInt(mode, 8);
     p = path.resolve(p);