[CLI] Throw better error messages

[peterjuras]

Summary

Errors are currently thrown without actionable error messages and hints on how to fix them. The goal should be to provide an actionable description and path to fix the error from occurring again.

Locations for errors with improvement potential:

• src/sync/merge-files.ts#123
• src/create/index.ts#100
• src/sync/gather-extended-templates.ts#29
• src/sync/merge-dependencies.ts#44
• src/sync/merge-files.ts#32
• src/sync/merge-files.ts#124
• src/sync/merge-files.ts#134
• src/util/get-context.ts#13