File loading fix (#36071)

https://github.com/grpc/grpc/pull/35856 was delayed because the API involved was being removed in https://github.com/grpc/grpc/pull/35857

This PR is the work we discussed as followup in https://github.com/grpc/grpc/pull/35857
Namely, removing the assert that will crash grpc because of a file loading failure - we don't want this, and would rather just return an error (https://b.corp.google.com/issues/262285099, b/262240495

Closes #36071

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36071 from gtcooke94:file_loading_fix cc358fadce
PiperOrigin-RevId: 616895887

src/core/lib/gprpp/load_file.cc

@@ -32,6 +32,9 @@ namespace grpc_core {
 
 // Loads the content of a file into a slice. add_null_terminator will add a NULL
 // terminator if true.
+// This API is NOT thread-safe and requires proper synchronization when used by
+// multiple threads, especially when they can happen to be reading from the same
+// file.
 absl::StatusOr<Slice> LoadFile(const std::string& filename,
                                bool add_null_terminator) {
   unsigned char* contents = nullptr;
@@ -61,7 +64,6 @@ absl::StatusOr<Slice> LoadFile(const std::string& filename,
   bytes_read = fread(contents, 1, contents_size, file);
   if (bytes_read < contents_size) {
     gpr_free(contents);
-    GPR_ASSERT(ferror(file));
     error = absl::InternalError(
         absl::StrCat("Failed to load file: ", filename,
                      " due to error(fread): ", strerror(errno)));