oiiotool internals: convert all error and warning to fmt style (#3240)

Also tagged Strutil and ustring::fmtformat as 'nodiscard', to produce
warnings if people call it and ignore the results. (As was the case in
my own bug that I tracked down in the course of this patch.)

Author: lgritz

src/include/OpenImageIO/strutil.h

@@ -137,6 +137,7 @@ using ::fmt::sprintf;
 
 namespace fmt {
 template<typename Str, typename... Args>
+OIIO_NODISCARD
 inline std::string format(const Str& fmt, Args&&... args)
 {
 #if FMT_VERSION >= 70000

src/include/OpenImageIO/ustring.h

@@ -639,7 +639,7 @@ class OIIO_UTIL_API ustring {
     /// The formatting of the string will always use the classic "C" locale
     /// conventions (in particular, '.' as decimal separator for float values).
     template<typename... Args>
-    static ustring sprintf(const char* fmt, const Args&... args)
+    OIIO_NODISCARD static ustring sprintf(const char* fmt, const Args&... args)
     {
         return ustring(Strutil::sprintf(fmt, args...));
     }
@@ -651,7 +651,8 @@ class OIIO_UTIL_API ustring {
     /// The formatting of the string will always use the classic "C" locale
     /// conventions (in particular, '.' as decimal separator for float values).
     template<typename... Args>
-    static ustring fmtformat(const char* fmt, const Args&... args)
+    OIIO_NODISCARD static ustring fmtformat(const char* fmt,
+                                            const Args&... args)
     {
         return ustring(Strutil::fmt::format(fmt, args...));
     }

src/oiiotool/imagerec.cpp

@@ -206,7 +206,7 @@ ImageRec::read_nativespec()
     ustring uname(name());
     if (!m_imagecache->get_image_info(uname, 0, 0, u_subimages, TypeInt,
                                       &subimages)) {
-        errorf("file not found: \"%s\"", name());
+        errorfmt("file not found: \"{}\"", name());
         return false;  // Image not found
     }
     m_subimages.resize(subimages);
@@ -223,7 +223,7 @@ ImageRec::read_nativespec()
                 new ImageBuf(name(), s, m, m_imagecache, m_configspec.get()));
             bool ok = ib->init_spec(name(), s, m);
             if (!ok)
-                errorf("%s", ib->geterror());
+                errorfmt("{}", ib->geterror());
             allok &= ok;
             m_subimages[s].m_miplevels[m] = ib;
             m_subimages[s].m_specs[m]     = ib->spec();
@@ -245,7 +245,7 @@ ImageRec::read(ReadPolicy readpolicy, string_view channel_set)
     ustring uname(name());
     if (!m_imagecache->get_image_info(uname, 0, 0, u_subimages, TypeInt,
                                       &subimages)) {
-        errorf("file not found: \"%s\"", name());
+        errorfmt("file not found: \"{}\"", name());
         return false;  // Image not found
     }
     m_subimages.resize(subimages);
@@ -335,7 +335,7 @@ ImageRec::read(ReadPolicy readpolicy, string_view channel_set)
                                             &newchannelnames[0], false);
             }
             if (!ok)
-                errorf("%s", ib->geterror());
+                errorfmt("{}", ib->geterror());
 
             allok &= ok;
             // Remove any existing SHA-1 hash from the spec.