config reading: Add a note about use of .exists

std::path::Path::exists is wrong and should never be used.  But let's
deal with that on a future occasion, as it may be controversial and/or
too much risk of a breaking change.

src/cargo/util/context/mod.rs

@@ -1537,6 +1537,11 @@ impl GlobalContext {
         let possible = dir.join(filename_without_extension);
         let possible_with_extension = dir.join(format!("{}.toml", filename_without_extension));
 
+        // Arguably this is wrong, because it ignores errors.
+        // I.e. it is "error-prone", as discussed in the docs for `std::path::Path::exists`.
+        //
+        // Instead, if we got an error, we should bail, but perhaps
+        // that might be too much risk of being a breaking change.
         if possible.exists() {
             if warn {
                 // We don't want to print a warning if the version