Show helper text for minimum balance requirements

[bekkibolthouse]

As a user trying to send n Stellars when my balance is >n+1%, I expect the transaction to complete. However, I get an error message "path could not send full amount." As a user, I am left wondering why the path could not complete the transaction and do not know what the expected threshold is.

Please add helping text that assists the user in understanding the minimum balance requirements so that s/he can have a more successful user experience.

https://www.dropbox.com/s/wypv1aj6xd2yunq/path%20could%20not%20send%20full%20amount.mov?dl=0

[irisli]

Also similar to https://github.com/stellar/stellar-client/issues/842