Improve host error messages

[agocke]

This is mostly focused around when users install dotnet using a zip/tarball instead of an installer or package manager.

There are two issues:

• If the user hasn't installed dotnet to global isntall directory or set DOTNET_ROOT, then the error message should mention where it looked for dotnet (and didn't find it)
• If the user passes a native file, the error message is:

Could not execute because the specified command or file was not found.
Possible reasons for this include:

• You misspelled a built-in dotnet command.
• You intended to execute a .NET program, but dotnet-bin/Debug/net10.0/hello does not exist.
• You intended to run a global tool, but a dotnet-prefixed executable with this name could not be found on the PATH.

This should instead have an error message saying that the target file was found, but is not a .NET assembly.

[dotnet-policy-service]

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