Raise an error when generating random integers when Height option is nonpositive

[d-torrance]

This fixes #2089. Also, we now raise an error when the height is negative instead of silently taking the absolute value (which was undocumented). The proposed behavior also matches the current behavior for random QQ's, not to mention random(ZZ), which yells if the input isn't positive.

Before

i1 : random(ZZ^5, ZZ^5, Height => -20)

o1 = | 13 10 11 3  13 |
     | 8  1  12 13 11 |
     | 16 0  1  1  7  |
     | 9  5  8  9  8  |
     | 1  16 12 15 8  |

              5       5
o1 : Matrix ZZ  <-- ZZ

i2 : random(ZZ^5, ZZ^5, Height => 0)
Floating point exception (core dumped)

Process M2 exited abnormally with code 136

After

i1 : random(ZZ^5, ZZ^5, Height => -20)
stdio:1:6:(3): error: expected a positive height

i2 : random(ZZ^5, ZZ^5, Height => 0)
stdio:2:6:(3): error: expected a positive height