t.merge and t.rast.algebra: enhance error message when output stds already exists

[veroandreo]

If the output stds already exists, G7:t.merge stops with the following:

t.merge inputs=daily_zeros_apr2003_oct2016,wnv_indicator_eip13 output=eip13_noholes
Traceback (most recent call last):
  File "/home/veroandreo/software/grass7_trunk/dist.x86_64-pc-linux-gnu/scripts/t.merge", line 147, in <module>
    main()
  File "/home/veroandreo/software/grass7_trunk/dist.x86_64-pc-linux-gnu/scripts/t.merge", line 104, in main
    (stds.get_new_map_instance(None).get_type(), output_id))
  File "/home/veroandreo/software/grass7_trunk/dist.x86_64-pc-linux-gnu/etc/python/grass/script/core.py", line 663, in fatal
    raise ScriptError(msg)
grass.exceptions.ScriptError: Unable to merge maps into space time raster dataset <eip13_noholes@nuevo_lst> please use the overwrite flag.

Something very similar happens with G7:t.rast.algebra. Is it possible to make these messages nicer, so the user only gets the last part of it, i.e.: the error, as in other modules?

I use trunk r71485 in a Fedora25 box.

[neteler]

Ticket retargeted after milestone closed

[martinl]

All enhancement tickets should be assigned to 7.6 milestone.

[martinl]

Ticket retargeted after milestone closed

[martinl]

Ticket retargeted after milestone closed