Improve validation error message

[beeme1mr]

Errors in the flag manifest are not presented in a user-friendly way. Improve the output so users can more easily understand how to address the issue.

Current output:

Error: error loading flag manifest: error validating JSON schema: jsonschema: '/flags/offer-free-shipping' does not validate with file:///home/nonroot/github.com/open-feature/cli/docs/schema/v0/flag_manifest.json#/properties/flags/patternProperties/%5E.%7B1%2C%7D$/$ref/required: missing properties: 'flagType', 'defaultValue'
error loading flag manifest: error validating JSON schema: jsonschema: '/flags/offer-free-shipping' does not validate with file:///home/nonroot/github.com/open-feature/cli/docs/schema/v0/flag_manifest.json#/properties/flags/patternProperties/%5E.%7B1%2C%7D$/$ref/required: missing properties: 'flagType', 'defaultValue'
Usage:
  openfeature generate react [flags]

Flags:
  -h, --help   help for react

Global Flags:
      --flag_manifest_path string   Path to the flag manifest.
      --output_path string          Output path for the codegen

Using the following manifest:

{
  "$schema": "https://flagd.dev/schema/v0/flags.json",
  "flags": {
    "offer-free-shipping": {
      "state": "ENABLED",
      "variants": {
        "on": true,
        "off": false
      },
      "defaultVariant": "on"
    },
    "sticky-header": {
      "state": "ENABLED",
      "variants": {
        "on": true,
        "off": false
      },
      "defaultVariant": "on",
      "targeting": {
        "if": [{ "==": [{ "var": "size" }, "sm"]}, "on", null ]
      }
    },
    "use-distributed-db": {
      "state": "ENABLED",
      "variants": {
        "on": true,
        "off": false
      },
      "targeting": {
        "fractional": [
          ["on", 0],
          ["off", 100]
        ]
      },
      "defaultVariant": "off"
    },
    "use-secure-protocol": {
      "state": "ENABLED",
      "variants": {
        "on": true,
        "off": false
      },
      "defaultVariant": "off"
    }
  }
}