chore: UX improvemnts on omit args & error handling

[wey-gu]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number: #162

Description:

• only if 0 arguments are provided, localhostl:9669 with root user will be used
• use exit or Fatal instead of Panic when any abnormal occurred

❯ ./nebula-console
Notice: Defaulting to localhost (127.0.0.1) with port 9669 using credentials (username: root, password: nebula).

Welcome!

(root@nebula) [(none)]>

❯ ./nebula-console -addr 10.1.1.168 -port 9669
Error: Invalid flags provided: the following required flags are missing or invalid: username
Tip: Use the --help option for guidance on correct flag usage.

❯ ./nebula-console -addr 10.1.1.168
Error: Invalid flags provided: the following required flags are missing or invalid: port, username
Tip: Use the --help option for guidance on correct flag usage.

Before, it's like this:

2024/03/15 08:29:13 Error: argument port is missed!
panic: Error: argument port is missed!

goroutine 1 [running]:
log.Panicf(0x7638b8, 0x1f, 0x0, 0x0, 0x0)
	/opt/hostedtoolcache/go/1.16.4/x64/src/log/log.go:361 +0xc5
main.validateFlags()
	/home/runner/work/nebula-console/nebula-console/main.go:467 +0x174
main.main()
	/home/runner/work/nebula-console/nebula-console/main.go:500 +0x10b