pgsql: Improve error message for MaxAllocSize overrun in accumArrayResu

Improve error message for MaxAllocSize overrun in accumArrayResult.

Before, if you went past about 64M array elements in array_agg() and
allied functions, you got a generic "invalid memory alloc request
size" error.  This patch replaces that with "array size exceeds the
maximum allowed", which seems more user-friendly since it points you
to needing to reduce the size of your array result.  (This is the
same error text you'd get from construct_md_array in the event of
overrunning the maximum physical size for the finished array.)

Per question from Shaozhong Shi.  Since this hasn't come up often,
I don't feel a need to back-patch.

Discussion: https://postgr.es/m/CA+i5JwYtVS9z2E71PcNKAVPbOn4R2wuj-LqbJsYr_XOz73q7dQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b8d3dae00f6c49d0c41abd5b36e2588a81abe5be

Modified Files
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src/backend/utils/adt/arrayfuncs.c | 6 ++++++
1 file changed, 6 insertions(+)