[BUGFIX] Low temp check: heat up extruder if below min_extrude_temp

[mjonuschat]

In the case that the extruder target temp is >0 but below the min extrude temp the current check fails to heat up the nozzle to a temperature where it's possible to oxtrude.

How to reproduce:

1. Set extruder temp to 150C
2. Try to load / unload filament
3. No heat up will be performed / error about extruder temp too low will be shown.