enh: Change the default value of `debug` to a FALSE-like sentinel value

[schloerke]

There are multiple reports of confusion around debug and the default error handler.

• Errors are displayed locally when interactive()
• Errors are not displayed when deployed

Default debug value:

plumber/R/plumber.R

Lines 1338 to 1340 in 7f51e24

	default_debug <- function() {
	interactive()
	}

Proposal

Change the default value of debug to behave like a FALSE value.

To maintain least surprise (default to same locally, display a message with instructions on how to turn in on:

Error reporting has been turned off by default. See `pr_set_debug` for more details. 
To disable this message, setting a debug value.

Tasks

• Change the default value of debug to behave like a FALSE value.
• Add rlang::inform(.frequency="once", .frequency_id="pr_set_debug_message") message.
  • This should only be displayed if rlang::is_interactive() is TRUE.
• Add #* @plumber example within pr_set_debug() docs to show to to set the value within a plumbed file
• News entry of changed behavior

[thomasp85]

Would NA or NULL not be sufficient sentinels along with the proposed message. These are standard sentinels for capturing lack of user action

I think conjuring up a new sentinel that behaves FALSEy but isn't false (so we can determine if a user has set it specifically to FALSE) could lead to just as much confusion

[schloerke]

NULL would be great! I believe this would allow for us to know when to display the inform message.

({plumber} hasn't really used NA when NULL can be used.)