Improve UX for entering the initial deposit amount in swap

[ralph-pichler]

Right now if the user experience for entering the initial deposit amount on the first start with Swap enabled is subpar.

A few things that should improve:

• The error for entering an invalid number is Conversion error while reading user input: strconv.ParseInt: parsing "": invalid syntax
• Also swarm instantly terminates in that case
• The input is in wei leading to huge numbers to enter

[Eknir]

@santicomp2014