Geometry error string is not shared

Instead move it to a private QgsGeometry member. It's copied
with QgsGeometry, but not the underlying private geometry
data.

This means the error string can be changed without detaching
(and copying) the geometry data.

Also make greater use of error string for all use of GEOS
functions.

python/core/geometry/qgsgeometry.sip

@@ -1292,10 +1292,11 @@ Returns an extruded version of this geometry.
  :rtype: int
 %End
 
-    QString error() const;
+    QString lastError() const;
 %Docstring
- Returns an error string referring to an error that was produced
- when this geometry was created.
+ Returns an error string referring to the last error encountered
+ either when this geometry was created or when an operation
+ was performed on the geometry.
 
 .. versionadded:: 3.0
  :rtype: str