Place excetion class in separate header and compilation unit.

The library shall produce more comprehensible error output. To achieve this,
it will be necessary to compile error message string by writing to string
streams and passing the resulting stream to a reporting function or exception.
All preprocessor macros which will make use of string streams are placed in
error.h and are used only internally in the library. A library user, however,
might need to access the definition of the execption class in case he likes to
catch and handle exceptions. The only way to keep the string stream overhead
away from users is to place the declaration of the exception class in a
separate header. This only confronts users with the additional definition of
strings, which is necessary for exceptions to work as intended.