[error] throw Error instead of CutesError

because classes has Error as prototype are not recognized as Error by
QJSValue.isError()

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[test] tracing on a critical level

To avoid error level tracing set by default, it can be changing by
providing number to ./run or by setting CUTES_DEBUG env var

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
more unit tests for os and subprocess

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>