fastboot: Improve error reporting on 'getvar partition-{size, type}'

fastboot: Improve error reporting on 'getvar partition-{size, type}'

Currently U-Boot reports the same error message in all below cases:
[A] host> fastboot getvar partition-type
[B] host> fastboot getvar partition-size
[C] host> fastboot getvar partition-type:
[D] host> fastboot getvar partition-size:
[E] host> fastboot getvar partition-type:<invalid-part>
[F] host> fastboot getvar partition-size:<invalid-part>

The message looks like:
host> fastboot getvar partition-size:
getvar:partition-size: FAILED (remote: partition not found)
Finished. Total time: 0.003s

Be more user friendly and output:
 - "partition not given" for [A-D]
 - "partition not found" for [E-F]

Fixes: f73a7df984a9 ("net: fastboot: Merge AOSP UDP fastboot")
Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Acked-by: Alex Kiernan <alex.kiernan@gmail.com>