various: do not propagate exception through dtor

Coverity 189689, 189690, 189692, 189695

src/common/http_connection.h

@@ -55,7 +55,8 @@ public:
   {
     if (m_ok)
     {
-      mp_http_client->disconnect();
+      try { mp_http_client->disconnect(); }
+      catch (...) { /* do not propagate through dtor */ }
     }
   }
 

src/wallet/wallet2.cpp

@@ -815,7 +815,12 @@ wallet_keys_unlocker::~wallet_keys_unlocker()
 {
   if (!locked)
     return;
-  w.encrypt_keys(key);
+  try { w.encrypt_keys(key); }
+  catch (...)
+  {
+    MERROR("Failed to re-encrypt wallet keys");
+    // do not propagate through dtor, we'd crash
+  }
 }
 
 wallet2::wallet2(network_type nettype, uint64_t kdf_rounds, bool unattended):