Wallet should still work if oasismonitor.com is down

[matevz]

If the external service such as oasismonitor.com is down, wallet should still offer basic functionality like fetching the wallet balance, receiving and sending ROSE:

• [x] Make a current full-page fatal error just a warning.
• [ ] Find out which functionalities don't require oasismonitor.com and gracefully hide functionalities that cannot work.
• [ ] Make errors regarding external services more descriptive. Current messages are cryptic and don't tell you that oasismonitor.com is down.