Fix watch task hanging with no directories

[RadicalZephyr]

Description

A check was added to the watch task's startup phase for whether the list of srcdirs is non-empty. If it is found to be empty, the task now exits with an error message.

Motivation and Context

Fixes #723.

How Has This Been Tested?

I was unable to test this at all because I have not yet been able to build and run my modified version of boot.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.