Improve error on interruption of GEOS-implemented functions

[strk]

When a GEOS implemented function is interrupted by user request or timeout, the error message that comes out is always something like:

ERROR:  GEOSBuffer: InterruptedException: Interrupted!

Instead, the PostgreSQL error message on interruption is more detailed about the event that triggered interruption, either:

ERROR:  canceling statement due to statement timeout

or

ERROR:  canceling statement due to user request

It would be nice to have the same detail when interrupting GEOS functions.

[strk]

r13104 implements this in trunk, not sure how worth it'd be to backport to 2.1 (2.1.0 was the first version adding interruptibility for GEOS functions, as long as GEOS-3.4+ was present at PostGIS build time). What do you think ?