Refactor help and error handling for op/open-preview.sh

[apolopena]

Problem this feature will solve

• -h --help text is not as clear as it could be.
• -h --help textshould be inline to the script not in a file outside the script.
• There is no error handling for the following edge cases:
  • no gp binary
  • invalid option
  • leading slash in the PATH
  • default port cannot be derived

Background

Proposed Solution

Clarify - -h --help text and put it inline to the script.

• Add error handling for the following edge cases:
  • no gp binary
  • invalid option
  • leading slash in the PATH
  • default port cannot be derived

Constraints and Assumptions

<! -- Any constraints and/or assumption relevant for the development and use of this feature? -->

Alternatives or Workarounds

Figure out the options yourself by reading the code in open-preview.sh

Additional context

Add any other context or screenshots about the feature request here.