Running some tests out-of-tree which pass a 'binary' argument
to start_node can result in the default 'bitcoind' binary
being unavailable. This can also happen if it is just deleted
and the user forgot to rebuild it.
This patch makes tests emits a friendlier message than the
huge "FileNotFound" traceback dump.
Now the test should report to the user:
Unable to locate bitcoind binary (./src/bitcoind).
Try specifying using --testbinary option, or set BITCOIND environment variable.
This offers two ways of passing the information about the correct
binary location.
A new framework self-test was added in `bitcoind_config_problems.py`,
to test the error messages related to bitcoind configuration problems.