Error message from required field should include prefix if specified.

[wfernandes]

If we have a prefix called "env_config" and the following struct tags for property "Foo", we should expect to see an error message stating that the required env var "ENV_CONFIG_BAR" is missing and not just "BAR" since that is the environment variable being looked up.

# With prefix configured as "env_config", the variable being looked up is "ENV_CONFIG_BAR".
type Spec struct {
  Foo string `envconfig:"bar" required:"true"`
}

Currently, the error message being reported is required key BAR missing value. It should be required key ENV_CONFIG_BAR missing value since that's the variable being looked up.

[mgerasimchuk]

@kelseyhightower please have a look

also faced this problem and @wfernandes solution works totally well for me

Would be great to merge it