Broken Cancel in sign node/cert view when fields invalid

[pwalski]

What:
Make Cancel button work even when form/field values in view of Signing node/certificate are invalid.

Why:
While creating new certificate I placed private key as a pub key and when I went into signing phase it did not let me to Sign (which is expected), but it also did not let me to Cancel (which was not expected). It does not let me Cancel this view when its field values are invalid. Only Esc works.

Originally posted by @pwalski in golemfactory/yagna#2542 (comment)