STYLE: Replace "Unknown" default for exception location with {}

It is preferable not duplicate a literal string so many times (following the DRY
principle). Moreover, it appears sufficient to use an empty string value (`{}`),
to indicate that the location of an exception is unspecified.

Note that an unspecified location may be "unknown", but instead, it may also be
"irrelevant", or "hidden". So it may depend on the application.

Triggered by a discussion with Bradley Lowekamp at
#5266 (comment)

Author: N-Dekker

Modules/Core/Common/include/itkExceptionObject.h

@@ -59,7 +59,7 @@ class ITKCommon_EXPORT ExceptionObject : public std::exception
   explicit ExceptionObject(std::string  file,
                            unsigned int lineNumber = 0,
                            std::string  desc = "None",
-                           std::string  loc = "Unknown");
+                           std::string  loc = {});
 
   /** Copy-constructor. */
   ExceptionObject(const ExceptionObject &) noexcept = default;

Modules/IO/ImageBase/include/itkImageFileReaderException.h

@@ -40,7 +40,7 @@ class ITKIOImageBase_EXPORT ImageFileReaderException : public ExceptionObject
   ImageFileReaderException(std::string  file,
                            unsigned int line,
                            std::string  message = "Error in IO",
-                           std::string  loc = "Unknown")
+                           std::string  loc = {})
     : ExceptionObject(std::move(file), line, std::move(message), std::move(loc))
   {}
 

Modules/IO/ImageBase/include/itkImageFileWriter.h

@@ -43,7 +43,7 @@ class ITKIOImageBase_EXPORT ImageFileWriterException : public ExceptionObject
   ImageFileWriterException(std::string  file,
                            unsigned int line,
                            std::string  message = "Error in IO",
-                           std::string  loc = "Unknown")
+                           std::string  loc = {})
     : ExceptionObject(std::move(file), line, std::move(message), std::move(loc))
   {}
 

Modules/IO/MeshBase/include/itkMeshFileReaderException.h

@@ -44,7 +44,7 @@ class ITKIOMeshBase_EXPORT MeshFileReaderException : public ExceptionObject
   MeshFileReaderException(std::string  file,
                           unsigned int line,
                           std::string  message = "Error in IO",
-                          std::string  loc = "Unknown");
+                          std::string  loc = {});
 };
 } // end namespace itk
 

Modules/IO/MeshBase/include/itkMeshFileWriterException.h

@@ -44,7 +44,7 @@ class ITKIOMeshBase_EXPORT MeshFileWriterException : public ExceptionObject
   MeshFileWriterException(std::string  file,
                           unsigned int line,
                           std::string  message = "Error in IO",
-                          std::string  loc = "Unknown");
+                          std::string  loc = {});
 };
 } // end namespace itk
 

Modules/Numerics/FEM/include/itkFEMException.h

@@ -49,7 +49,7 @@ class ITK_ABI_EXPORT FEMException : public itk::ExceptionObject
    * you should use __FILE__ and __LINE__ macros to specify file name
    * and line number.
    */
-  FEMException(std::string file, unsigned int lineNumber, std::string location = "Unknown");
+  FEMException(std::string file, unsigned int lineNumber, std::string location = {});
 
   /** Virtual destructor needed for subclasses. Has to have empty throw(). */
   ~FEMException() noexcept override;