Reduce a bit the size of getHT

Factoring common code parts outside of a template
is apparently a good practise to reduce code duplication
(and the size of openmw by around 0.5%),
and should improve a bit the performances,
since the whole `std::to_string` * 2 + string concatenation
dance results in quite a lot of code, preventing inlining on my machine.

components/esm/esmreader.cpp

@@ -198,7 +198,7 @@ void ESMReader::skipHSubSize(int size)
 {
     skipHSub();
     if (static_cast<int> (mCtx.leftSub) != size)
-        fail("skipHSubSize() mismatch");
+        reportSubSizeMismatch(mCtx.leftSub, size);
 }
 
 void ESMReader::skipHSubUntil(const char *name)

components/esm/esmreader.hpp

@@ -134,11 +134,7 @@ public:
   {
       getSubHeader();
       if (mCtx.leftSub != sizeof(X))
-      {
-          fail("getHT(): subrecord size mismatch,requested "
-                  + std::to_string(sizeof(X)) + ", got"
-                      + std::to_string(mCtx.leftSub));
-      }
+          reportSubSizeMismatch(sizeof(X), mCtx.leftSub);
       getT(x);
   }
 
@@ -261,6 +257,13 @@ public:
   size_t getFileSize() const { return mFileSize; }
 
 private:
+  [[noreturn]] void reportSubSizeMismatch(size_t want, size_t got) {
+          fail("subrecord size mismatch, requested " +
+                  std::to_string(want) +
+                  ", got" +
+                  std::to_string(got));
+  }
+
   void clearCtx();
 
   Files::IStreamPtr mEsm;