test: don't exit on flag parsing error

[achille-roussel]

This PR modifies the main package and its tests to not call os.Exit on invalid command line arguments, which makes it possible to invoke main.Root from tests.

This has a few advantages:

• we don't need to build the binary to run the tests, so a simple go test captures the latest changes without requiring an extra step to rebuild timecraft
• we can use pprof with go test to profile the program execution (it was masked when invoking timecraft as a separate binary)
• the tests run faster because the baseline cost of a function call is much lower than creating a new process
• overall the code is better structured, there is no secret escape of from the control flow to terminate the program, functions have to return or panic (so defers are always run)
• go test can compute the test coverage of the top-level package

[pelletier]

nice!