Make error messages more consistent

[ehmicky]

This makes error.message more consistent.

execa errors in the following cases:
a) exit code is defined and is not 0
b) child process error event, or stdin error event
c) signal termination (including cancel())
d) timeout
e) stream error (separate issue: #228)

a) c) d) e) look consistent. However b):

• prints stdout and stderr, as opposed to the others. This can also lead to unexpectedly (very) long error messages, which might create issues for some users. error.stdout and error.stderr can be used instead.
• does not print the command, as opposed to the others.

This adjusts b) error message to the same format used by the other errors.

It turns out to cover this, I just had to remove unit tests but did not need to add new ones (could not think of one not already covered).

[ehmicky]

CI is now passing.