[mathiasrw]

Closes #728

Change

• error() counts the errors of the run, and engine() resolves with the number of errors. The engine does not touch the exit code.
• The CLI sets process.exitCode = 1 when the engine resolves with errors. The run goes on, and the process ends with 1 when the last file is done. --halt still stops at the first error.
• --quiet and --quiet-total only hide the message. The exit code is the same.

Error	Exit code	With --halt
0 files found	1	1
The file can not be read (EACCES on open)	1	1
The backup can not be made (folder is read-only)	1	1
Two files given, one is changed and one fails	1	1
All goes well	0	0

Tests

• rr() in test/js/helper.js gives exit: 1 when the engine resolves with errors, like the CLI.
• Tests that expected exit: null after an error now expect exit: 1.
• The --halt tests show what --halt changes: the other files are left alone with it, and handled without it.
• The two exit code tests for -q and -Q in test/cli/run.ai.sh are turned on.

Later

• Let the engine return a status report and the CLI own the process #742: the engine resolves with a full status in place of the number of errors, and a panic in place of process.exit().