Print a message when a command receives a signal

Receiving a signal is normally indicative of violent termination, so the
subcommand can't be relied upon to have printed some status information. As a
result, signals now have some extra errors printed to stderr when they fail.

Closes #261. The actual signal is still a bug, but it's an upstream rust bug.
auto merge of #265 : alexcrichton/cargo/issue-261, r=wycats

Receiving a signal is normally indicative of violent termination, so the
subcommand can't be relied upon to have printed some status information. As a
result, signals now have some extra errors printed to stderr when they fail.

Closes #261. The actual signal is still a bug, but it's an upstream rust bug.