Add key name to "Value not set" error message (#2149)

The error typically occurs when writeMetadata() is called. It's easier
to find the problem if you know which datum hasn't had a value set.

src/error.cpp

@@ -23,7 +23,7 @@ namespace
         N_("Invalid record name '%1'"),                  // kerInvalidRecord %1=record name
         N_("Invalid key '%1'"),                          // kerInvalidKey %1=key
         N_("Invalid tag name or ifdId `%1', ifdId %2"),  // kerInvalidTag %1=tag name, %2=ifdId
-        N_("Value not set"),                             // kerValueNotSet
+        N_("%1 value not set"),                          // kerValueNotSet %1=key name
         N_("%1: Failed to open the data source: %2"),    // kerDataSourceOpenFailed %1=path, %2=strerror
         N_("%1: Failed to open file (%2): %3"),          // kerFileOpenFailed %1=path, %2=mode, %3=strerror
         N_("%1: The file contains data of an unknown image type"),  // kerFileContainsUnknownImageType %1=path

src/exif.cpp

@@ -205,7 +205,7 @@ namespace Exiv2 {
     const Value& Exifdatum::value() const
     {
         if (!value_)
-            throw Error(ErrorCode::kerValueNotSet);
+            throw Error(ErrorCode::kerValueNotSet, key());
         return *value_;
     }
 

src/iptc.cpp

@@ -131,7 +131,7 @@ namespace Exiv2 {
     const Value& Iptcdatum::value() const
     {
         if (!value_)
-            throw Error(ErrorCode::kerValueNotSet);
+            throw Error(ErrorCode::kerValueNotSet, key());
         return *value_;
     }
 

src/xmp.cpp

@@ -369,7 +369,7 @@ namespace Exiv2 {
     const Value& Xmpdatum::value() const
     {
         if (!p_->value_)
-            throw Error(ErrorCode::kerValueNotSet);
+            throw Error(ErrorCode::kerValueNotSet, key());
         return *p_->value_;
     }