Panic when fuse fails to start

[enocom]

When the fuse binary is missing, the user doesn't have read access, user_allow_other isn't set, etc., the Proxy will panic instead of reporting the error and exiting gracefully.

Stacktrace:

enocom@instance-1:~$ ./cloud-sql-proxy --fuse /cloudsql
2023/10/31 19:42:46 Authorizing with Application Default Credentials
2023/10/31 19:42:46 The proxy has started successfully and is ready for new connections!
/usr/bin/fusermount: option allow_other only allowed if 'user_allow_other' is set in /etc/fuse.conf
2023/10/31 19:42:46 The proxy has encountered a terminal error: FUSE mount failed: "/cloudsql": fusermount exited with code 256
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xbc3052]

goroutine 1 [running]:
github.com/hanwen/go-fuse/v2/fuse.(*Server).Unmount(0x658bb9?)
        /workspace/GOPATH/pkg/mod/github.com/hanwen/go-fuse/[email protected]/fuse/server.go:128 +0x12
github.com/GoogleCloudPlatform/cloud-sql-proxy/v2/internal/proxy.(*Client).unmountFUSE(0x0?)
        /workspace/internal/proxy/proxy_other.go:183 +0x85
github.com/GoogleCloudPlatform/cloud-sql-proxy/v2/internal/proxy.(*Client).Close(0xc000564280)
        /workspace/internal/proxy/proxy.go:647 +0x5c
github.com/GoogleCloudPlatform/cloud-sql-proxy/v2/cmd.runSignalWrapper.func7()
        /workspace/cmd/root.go:821 +0x2d
github.com/GoogleCloudPlatform/cloud-sql-proxy/v2/cmd.runSignalWrapper(0xc000632780)
        /workspace/cmd/root.go:911 +0x160c
github.com/GoogleCloudPlatform/cloud-sql-proxy/v2/cmd.NewCommand.func3(0xc00057e300, {0xc000500fa0, 0x7, 0xf6388a})
        /workspace/cmd/root.go:361 +0x17
github.com/spf13/cobra.(*Command).execute(0xc00057e300, {0xc00003e250, 0x2, 0x2})
        /workspace/GOPATH/pkg/mod/github.com/spf13/[email protected]/command.go:940 +0x87c
github.com/spf13/cobra.(*Command).ExecuteC(0xc00057e300)
        /workspace/GOPATH/pkg/mod/github.com/spf13/[email protected]/command.go:1068 +0x3a5
github.com/spf13/cobra.(*Command).Execute(...)
        /workspace/GOPATH/pkg/mod/github.com/spf13/[email protected]/command.go:992
github.com/GoogleCloudPlatform/cloud-sql-proxy/v2/cmd.Execute()
        /workspace/cmd/root.go:76 +0x25
main.main()
        /workspace/main.go:25 +0xf