cmd: Add fail-fast approach in case of incorrect config file

[leszko]

What does this pull request do? Explain your changes. (required)
Fail fast when there is an error in the Livepeer config file.

Specific updates (required)
N/A

How did you test each of these updates (required)

1. Non-existing config file

$ ./livepeer -config nonExistingFile.conf
F0120 11:58:24.943412   42834 livepeer.go:171] Error using config file: open nonExistingFile.conf: no such file or directory

2. Config file with a flag that was not defined

$ cat livepeer.conf 
reward false malformed
$ ./livepeer -config livepeer.conf 
F0120 11:59:23.205466   42904 livepeer.go:171] Error using config file: error setting flag "reward" from config file: parse error

3. Malformed config file

$ cat livepeer.conf 
nonExistingFlag 123
reward false
$ ./livepeer -config livepeer.conf 
F0120 11:59:52.497457   42961 livepeer.go:171] Error using config file: config file flag "nonExistingFlag" not defined in flag set

Does this pull request close any open issues?
fix #2193

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[yondonfu]

LGTM. Just a minor question - feel free to address before merging if you agree.

[yondonfu]

IIUC this error message will be not be accurate if CLI flags are used instead of a config file since ff.Parse() does not necessarily parse a config file if one is not provided and CLI flags are passed instead. Perhaps a more descriptive error message would be just: "Error parsing config"? WDYT?

[leszko]

Right, updated in 3c0e0d3