Replace all String-based error messages with structured error types

[RyanGlScott]

Generally speaking, elf-edit adheres to the convention of using structured data types to represent its error messages, such as with the DynamicError data type. Unfortunately, there are still some parts of elf-edit that use raw Strings to represent errors:

• elf-edit/src/Data/ElfEdit/Dynamic.hs

  Lines 324 to 329 in 40ffaac

  	-- | Parses a linked list
  	gnuLinkedList :: (B.ByteString -> Get a) -- ^ Function for reading.
  	-> ElfData
  	-> Int -- ^ Number of entries expected.
  	-> B.ByteString -- ^ Buffer to read.
  	-> Either String [a]

• elf-edit/src/Data/ElfEdit/Dynamic.hs

  Lines 632 to 633 in 40ffaac

  	-- | Get values of DT_NEEDED entries
  	dynNeeded :: forall w . DynamicSection w -> VirtAddrMap w -> Either String [B.ByteString]

• elf-edit/src/Data/ElfEdit/HighLevel/GOT.hs

  Lines 55 to 58 in fb4699a

  	-- | Attempt to convert a section to a GOT.
  	elfSectionAsGOT :: (Bits w, Num w)
  	=> ElfSection w
  	-> Either String (ElfGOT w)

• elf-edit/src/Data/ElfEdit/HighLevel/Get.hs

  Lines 96 to 101 in fb4699a

  	transShdr :: Integral w
  	=> B.ByteString -- ^ Contents fof file.
  	-> B.ByteString -- ^ String table for sectionnames
  	-> Word16 -- ^ Index of section
  	-> Shdr Word32 w
  	-> Either String (FileRange w, ElfSection w)

• elf-edit/src/Data/ElfEdit/Relocations/Common.hs

  Lines 213 to 218 in fb4699a

  	-- | Return relocation entries from byte string.
  	decodeRelEntries :: forall tp
  	. IsRelocationType tp
  	=> ElfData -- ^ Endianess of encodings
  	-> B.ByteString -- ^ Relocation entries
  	-> Either String [RelEntry tp]

• elf-edit/src/Data/ElfEdit/Relocations/Common.hs

  Lines 247 to 252 in fb4699a

  	-- | Return relocation entries from byte string.
  	decodeRelaEntries :: forall tp
  	. IsRelocationType tp
  	=> ElfData -- ^ Endianess of encodings
  	-> B.ByteString -- ^ Relocation entries
  	-> Either String [RelaEntry tp]

We should replace these uses of Either String with Either <structured error type>, where the specific structured error type is specific to the function in question. We might be able to use existing error types for some functions; for instance, dynNeeded could likely use Either DynamicError, just like other functions in Data.ElfEdit.Dynamic do.

I don't have a pressing need to fix this right now, especially since fixing this would require API changes. This does seem like a goal that we should work towards in the long term, however.