TgaImporter: use a better error message for RLE decoding failures.

src/MagnumPlugins/TgaImporter/Test/TgaImporterTest.cpp

@@ -434,7 +434,7 @@ void TgaImporterTest::rleTooLarge() {
     std::ostringstream out;
     Error redirectError{&out};
     CORRADE_VERIFY(!importer->image2D(0));
-    CORRADE_COMPARE(out.str(), "Trade::TgaImporter::image2D(): RLE data larger than advertised Vector(2, 3) pixels at byte 28\n");
+    CORRADE_COMPARE(out.str(), "Trade::TgaImporter::image2D(): RLE data at byte 28 contains 4 pixels but only 3 left to decode\n");
 }
 
 void TgaImporterTest::openMemory() {

src/MagnumPlugins/TgaImporter/TgaImporter.cpp

@@ -167,7 +167,7 @@ Containers::Optional<ImageData2D> TgaImporter::doImage2D(UnsignedInt, UnsignedIn
                 return Containers::NullOpt;
             }
             if(count*pixelSize > dstPixels.size()) {
-                Error{} << "Trade::TgaImporter::image2D(): RLE data larger than advertised" << size << "pixels at byte" << (srcPixels.data() - _in.data());
+                Error{} << "Trade::TgaImporter::image2D(): RLE data at byte" << (srcPixels.data() - _in.data()) << "contains" << count << "pixels but only" << dstPixels.size()/pixelSize << "left to decode";
                 return Containers::NullOpt;
             }