[lld][WebAssembly] update error to continue for R_WASM_FUNCTION_INDEX_I32 (llvm#162403)

with some builds we're getting
`[libcxx/include/optional:874](libcxx/include/optional): libc++
Hardening assertion this->has_value() failed: optional operator* called
on a disengaged value` since error() adds the errmsg into the stream and
continues, but given it's an unsupported relocation type it eventually
crashes.

Given that I see that we're already using Fatal() in some of the other
places where it hits unsupported relocation type, my uneducated guess is
that this should be fine.

Author: kevinwkt

lld/wasm/InputChunks.cpp

@@ -439,9 +439,11 @@ bool InputChunk::generateRelocationCode(raw_ostream &os) const {
     if (!requiresRuntimeReloc)
       continue;
 
-    if (!isValidRuntimeRelocation(rel.getType()))
+    if (!isValidRuntimeRelocation(rel.getType())) {
       error("invalid runtime relocation type in data section: " +
             relocTypetoString(rel.Type));
+      continue;
+    }
 
     uint64_t offset = getVA(rel.Offset) - getInputSectionOffset();
     LLVM_DEBUG(dbgs() << "gen reloc: type=" << relocTypeToString(rel.Type)