Fix flycheck-error-level-interesting-p

According to its docstring, `flycheck-error-level-interesting-p` should
return true if there are no errors as _or more severe than_
`flycheck-navigation-minimum-level`. Currently, however, it only checks
if there are any errors exactly as severe as
`flycheck-navigation-minimum-level` (via
`flycheck-has-current-errors-p`). This patch:

* improves the documentation of
  * `flycheck-has-current-errors-p`
  * `flycheck-has-errors-p`
* introduces two new functions
  * `flycheck-has-current-errors-atleast-p`
  * `flycheck-has-errors-atleast-p`
* Changes `flycheck-error-level-interesting-p` to use
  `flycheck-has-current-errors-atleast-p`, instead of its "exact"
  sibling, bringing its actual behavior in line with its documented
  behavior

flycheck.el

@@ -4652,17 +4652,34 @@ level."
  (flycheck-has-max-errors-p flycheck-current-errors level))
 
 (defun flycheck-has-errors-p (errors level)
- "Determine if there are any ERRORS with LEVEL."
+ "Determine if there are any ERRORS with exactly LEVEL."
  (seq-some (lambda (e) (eq (flycheck-error-level e) level)) errors))
 
+(defun flycheck-has-errors-atleast-p (errors level)
+ "Determine if there are any ERRORS with at least LEVEL."
+ (seq-some (lambda (e)
+ (>= (flycheck-error-level-severity (flycheck-error-level e))
+ (flycheck-error-level-severity level)))
+ errors))
+
 (defun flycheck-has-current-errors-p (&optional level)
- "Determine if the current buffer has errors with LEVEL.
+ "Determine if the current buffer has any errors with exactly LEVEL.
 
-If LEVEL is omitted if the current buffer has any errors at all."
+If LEVEL is omitted, then return whether the current buffer has any errors at
+all."
  (if level
  (flycheck-has-errors-p flycheck-current-errors level)
  (and flycheck-current-errors t)))
 
+(defun flycheck-has-current-errors-atleast-p (&optional level)
+ "Determine if the current buffer has any errors with at least LEVEL.
+
+If LEVEL is omitted, then return whether the current buffer has any errors at
+all."
+ (if level
+ (flycheck-has-errors-atleast-p flycheck-current-errors level)
+ (and flycheck-current-errors t)))
+
 
 ;;; Error overlays in the current buffer
 (defvar-local flycheck--last-overlay-index 0
@@ -4851,7 +4868,7 @@ no errors as or more severe than `flycheck-navigation-minimum-level'."
  (-if-let (min-level flycheck-navigation-minimum-level)
  (or (<= (flycheck-error-level-severity min-level)
  (flycheck-error-level-severity (flycheck-error-level err)))
- (not (flycheck-has-current-errors-p min-level)))
+ (not (flycheck-has-current-errors-atleast-p min-level)))
  t)))
 
 (defun flycheck-next-error-pos (n &optional reset)