livepeer_cli: Improve user feedback when specifying numeric values for some wizard options

[kparkins]

What does this pull request do? Explain your changes. (required)
This PR addresses issues outlined in Issue #1602 relating to poor user experience in the livepeer_cli when specifying numeric values for some wizard options (gas price, bonding, unbonding, and transfer amount). In particular, it attempts to provide better feedback to the user when invalid values are supplied.

Specific updates (required)

• Add prompt parameter to readBigInt in wizard.go and change to log an error when parsing an int fails.
• Update calls to readBigInt to pass a relevant prompt.

How did you test each of these updates (required)

• Ran all unit tests (PASSED)
• Tested the CLI manually for each of the affected options.

Does this pull request close any open issues?

• Fixes livepeer_cli gas price validation #1602
• Closing 1602 may mean a new issue should be created to follow-up on other wizard options that were not addressed.

Checklist:

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

[leszko]

Thanks the PR @kparkins . I think it'll be a good improment. Added a very minor comments, please take a look.

[leszko]

LGTM 👍

Thanks for the PR @kparkins