seth ls or seth send do nothing if default account not found

**Issue by [rolandkofler](https://github.com/rolandkofler)**
_Sunday Feb 04, 2018 at 12:05 GMT_
_Originally opened as https://github.com/dapphub/seth/issues/33_

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# actual behavior
if the default address `export ETH_FROM=0x...` is not found seth ls (and many other cmds) simply stop working, and you have to ^C them.

# should
if no valid default account is found an error msg should be thrown and the cmd should exit with an error code.

Start by reproducing `seth ls` and `seth send` with `ETH_FROM` set to an unavailable default account. Trace the `seth` command handling and verify that a missing account produces an error message and a nonzero exit code instead of hanging or requiring interruption.

Written by the indexing model from the issue text.