Initial checks to for avoidable errors

[gianfra-t]

Issue: #120

Added checks

• On initialization of the app, we check if all accounts in the signing-service have the corresponding amounts. We also check inherently that the service is responsive.
• When the user clicks Confirm, we now check:
  • There exists at least one stellar vault that has redeemable tokens for the expected output of Nabla (w/ a margin, if defined) minOutputAmount * margin.
  • A route (squidrouter) was found.

If one of the checks fails, the Confirm button will be disabled, a generic message will show on top of it and the user will need to reload to perform the checks again.

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