Fix errors when pages are missing from the cache

When the language directories are empty, the entire cache dir has
to be deleted in order for --update to work properly.
If the user manually deletes pages for some reason, the error messages
now tell them to run --clean-cache first.

src/cache.rs

@@ -301,8 +301,8 @@ impl<'a> Cache<'a> {
                 }
 
                 if result.is_empty() {
-                    Err(Error::new(
-                        "'pages.en' contains no platform directories. Please run 'tldr --update'.",
+                    Err(Error::messed_up_cache(
+                        "'pages.en' contains no platform directories.",
                     ))
                 } else {
                     // read_dir() order can differ across runs, so it's
@@ -421,7 +421,9 @@ impl<'a> Cache<'a> {
 
     fn print_basenames(mut pages: Vec<OsString>) -> Result<()> {
         if pages.is_empty() {
-            return Err(Error::new("no pages found. Please run 'tldr --update'."));
+            return Err(Error::messed_up_cache(
+                "no pages found, but the 'pages.en' directory exists.",
+            ));
         }
 
         // Show pages in alphabetical order.

src/error.rs

@@ -92,6 +92,13 @@ impl Error {
             .kind(ErrorKind::Download)
     }
 
+    pub fn messed_up_cache(e: &str) -> Self {
+        Error::new(format!(
+            "{e}\n\nThis should never happen, did you delete something from the cache?\n\
+            Please run 'tldr --clean-cache' followed by 'tldr --update' to redownload all pages."
+        ))
+    }
+
     /// Print the error message to stderr and return an appropriate `ExitCode`.
     pub fn exit_code(self) -> ExitCode {
         let _ = writeln!(io::stderr(), "{} {self}", "error:".red().bold());