Improve string representation of Assignment Error

[andreaskaris]

Requires go 1.18 or higher

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer (optional):

Here's the method in the playground:
https://go.dev/play/p/cKqAfUKCasJ?v=goprev

new format: Could not allocate IP in range: network: 192.168.1.0/24, start: 192.168.1.2, end: 192.168.1.10
old format: Could not allocate IP in range: ip: 192.168.1.2 / - 192.168.1.10 / range: net.IPNet{IP:net.IP{0xc0, 0xa8, 0x1, 0x0}, Mask:net.IPMask{0xff, 0xff, 0xff, 0x0}}

[coveralls]

Pull Request Test Coverage Report for Build 4596793331

• 0 of 3 (0.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 71.397%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/allocate/allocate.go	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
pkg/allocate/allocate.go	1	70.37%

Totals
Change from base Build 4596273673:	-0.1%
Covered Lines:	966
Relevant Lines:	1353

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💛 - Coveralls

[andreaskaris]

/retest