[fleet installer] Update (and fix) return code

[andrewlock]

Summary of changes

Make ErrorRemovingNativeLoaderFiles = 2 as it's a special case return code

Reason for change

This is the only return code other than 0 and 1 which currently matters to the fleet-installer, so moving it to 2 from 6 primarily for aesthetics.

Implementation details

Move the ErrorRemovingNativeLoaderFiles error code and explicitly set its value

[andrewlock]

Execution-Time Benchmarks Report ⏱️

Execution-time results for samples comparing the following branches/commits:

• This PR (6712)
• master

Execution-time benchmarks measure the whole time it takes to execute a program. And are intended to measure the one-off costs. Cases where the execution time results for the PR are worse than latest master results are shown in red. The following thresholds were used for comparing the execution times:

• Welch test with statistical test for significance of 5%
• Only results indicating a difference greater than 5% and 5 ms are considered.

Note that these results are based on a single point-in-time result for each branch. For full results, see the dashboard.

Graphs show the p99 interval based on the mean and StdDev of the test run, as well as the mean value of the run (shown as a diamond below the graph).

gantt
    title Execution time (ms) FakeDbCommand (.NET Framework 4.6.2) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (6712) - mean (69ms)  : 67, 71
     .   : milestone, 69,
    master - mean (70ms)  : 64, 76
     .   : milestone, 70,

    section CallTarget+Inlining+NGEN
    This PR (6712) - mean (1,002ms)  : 982, 1023
     .   : milestone, 1002,
    master - mean (1,001ms)  : 977, 1026
     .   : milestone, 1001,

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gantt
    title Execution time (ms) FakeDbCommand (.NET Core 3.1) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (6712) - mean (102ms)  : 100, 105
     .   : milestone, 102,
    master - mean (102ms)  : 100, 105
     .   : milestone, 102,

    section CallTarget+Inlining+NGEN
    This PR (6712) - mean (671ms)  : 657, 686
     .   : milestone, 671,
    master - mean (676ms)  : 661, 691
     .   : milestone, 676,

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gantt
    title Execution time (ms) FakeDbCommand (.NET 6) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (6712) - mean (90ms)  : 88, 91
     .   : milestone, 90,
    master - mean (89ms)  : 88, 91
     .   : milestone, 89,

    section CallTarget+Inlining+NGEN
    This PR (6712) - mean (633ms)  : 618, 647
     .   : milestone, 633,
    master - mean (633ms)  : 615, 651
     .   : milestone, 633,

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gantt
    title Execution time (ms) HttpMessageHandler (.NET Framework 4.6.2) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (6712) - mean (192ms)  : 188, 197
     .   : milestone, 192,
    master - mean (191ms)  : 188, 195
     .   : milestone, 191,

    section CallTarget+Inlining+NGEN
    This PR (6712) - mean (1,113ms)  : 1088, 1139
     .   : milestone, 1113,
    master - mean (1,113ms)  : 1088, 1138
     .   : milestone, 1113,

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gantt
    title Execution time (ms) HttpMessageHandler (.NET Core 3.1) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (6712) - mean (272ms)  : 267, 277
     .   : milestone, 272,
    master - mean (272ms)  : 266, 277
     .   : milestone, 272,

    section CallTarget+Inlining+NGEN
    This PR (6712) - mean (867ms)  : 829, 904
     .   : milestone, 867,
    master - mean (870ms)  : 834, 906
     .   : milestone, 870,

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gantt
    title Execution time (ms) HttpMessageHandler (.NET 6) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (6712) - mean (264ms)  : 260, 267
     .   : milestone, 264,
    master - mean (263ms)  : 259, 267
     .   : milestone, 263,

    section CallTarget+Inlining+NGEN
    This PR (6712) - mean (848ms)  : 817, 878
     .   : milestone, 848,
    master - mean (852ms)  : 813, 892
     .   : milestone, 852,

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