[yaml2elf] Replace error message by assert call in writeSectionContent methods

[yaml2elf] Replace error message by assert call in writeSectionContent methods

Now caller of ELFState::writeSectionContent() methods is responsible to check
a section type and selects an appropriate writeSectionContent method.
So unexpected section type inside writeSectionContent method indicates
a wrong usage of the method and should be guarded by assert.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236808 91177308-0d34-0410-b5e6-96231b3b80d8